Zilean is a champion that is often under-estimated.. to his advantage!
His abilities simply works great to get a kill on a champion that dares being a bit too careless. His ult can completely turn a gank around and starve the other teams of important kills (thus, important extra xp/gold). He gives your entire team 8% xp bonus and these advantages...

How does unlimited ghost spell sounds?
A champion that can perma-slow an enemy?
A guardian angel that only takes 30 seconds to recharge and can fill any champion's health at least 50% full?
Being able to push the 2 other lanes quickly while your team is busy with another one?

Some people will suggest you to choose ability power over cool-down, others will tell you that you're better off with cool-down reduction to spam your ult. Why should you have to choose when Zilean can take both AP and CD without any sacrifice? Zilean is both a terrific support and a frightening killing machine. With the right Masteries, Runes and items, you will discover the true power of this great Champion.

He is my favorite champion and he is a ton of fun to use. I felt like sharing my secrets with him so listen closely.

Pros
Good range
Great Harasser
Great Farmer
Great damage and kill opportunities early/mid game
Almost never runs out of mana late game
Extremely fast
Extremely annoying late game (for the other team)
Extremely fun to play
Probably one of the best, if not the best, support champion of the game

Cons
Very Team dependent
Very squishy
Gets targeted first against competent teams
Takes a while to get used to him
Very Kill Dependant if you want to carry
You will spam the chat often screaming ULT!!! I ULTED YOU, DON'T RUN DAMMIT!

No need to explain much here

What *I* get
Extra speed = no escape from your bombs and its a life saver. Why taking ghost while you have a skill that does the same thing? Main reason is to save mana early game and also I'm not a big fan of flash. Beside, you always want to keep a safe distance from your enemies and flash tends to make you have the reflex to teleport right next to them when using it offensively.
Prevents the enemy champion from healing and reduce their hp even more to give you the kill when that second bomb pops their head off


What not to get
You wont need it after early game and would be a waste
I don't see any use of this for Zilean since his is even better and can be used at will
You are not a jungler
The only thing this spell does well is make people leave the waiting room

9/0/21, I honestly have nothing else to add, who would build this champion otherwise?!

About the ability leveling order...
why do I level up Time warp and Rewind at the same time?
-Max Leveling Rewind will reduce its cooldown... and your already too low mana bar mid game
-Timewarp is a great escape mechanism that can save yourself and a teammate when in a desperate situation
-The extra speed allows you to push more efficiently
-The golem buff already reduces cooldowns but doesn't improve your speed doesn't it?

Magic penetration & Ability Power to make your even deadlier!

Lets have fun doing some simple math here. Your passive gives you 8% xp bonus, Your Masteries gives you 5% xp bonus, these runes gives you 6% xp bonus. You will have a total of 19% Xp Bonus for each minions/champions you kill allowing you to get your lvl 6 before anybody else in the game, if you get 2 or 3 kills early game, You can end up 5 levels ahead of the rest of your enemies. This can give you an insane advantage for the entire game!

Starter

You will be mana STARVING early game and will constantly have to save it. Mana regen is a MUST, Health potions are always good as well since you will only start with 400 hp.
Gold remaining: 15

Ideal first trip back to base

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Be careful and do your best to keep your enemies at a distance by hiding in a bush and harassing them. If you do this, You should be able to stay in your lane until you can purchase both these items.

Early game you will very quickly drain all your mana which makes this item a MUST since it will increase your mana with each spell you use in the future

You want to get extra movement speed since you are very squishy and can get taken down easily if you're too slow
Gold needed: 950

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Once you get this item, You are really starting to be in business, you still get aditional 4 mana for each spell you use, get aditional mana, and you get a 3% bonus of your max mana converted into your ability power

These boots, combined with your will make you EXTREMELY FAST which is very useful for Zilean. Now here comes your question, why not getting magic pen or cooldown.. etc etc instead..? Because as a Zilean, You want to be able to travel around the map very quickly. It allows you to..
-be an excellent multi lane pusher
Late game Zilean can push all 3 lanes in only a matter of seconds between 2 team fights, allowing you to constantly keep your minions at their tower and allow you to push.
-reach any teammate in need on the map very quickly
Having additional magic pen won't make as much as a difference if you can't reach your teammate in time won't it? Cooldown reduction is also useless since this build will eventually get to there so be patient!
-escape from pretty much any minor gank
With that much speed, you are nearly uncatchable for most champions unless they have a stun, and if they do, they better kill you before you're free from it else you'll be able to ult yourself or just run away. I rarely get killed by a gank with less than 3 people. These boots made me say "yoink!" so many times I now feel naked without them.

More ability power for your and , also gives you some very needed HP and even more mana for your personal enjoyment.

Now you start building up your cooldown reduction which will turn you into a plague for the other team, get this item as soon as you have enough gold for it, it will already greatly decrease your ult's cooldown and give you even more mana that will get converted into ability power (Total Cooldown 30%)

You can skip this step if you feel confident enought that you won't get hit too much to directly jump for more ability power, however I *rarely* skip this item since being a bit less squishy can make a big difference while you're being chased (Total Cooldown 35%)

This item brings you to your 40% cooldown reduction limit, it also gives you an additional 60 ability power with a POWERFUL active that can, at that point, remove nearly 50% of any champion's hp.

And this finishes your build, if you get to this point, you will be able to ult around every 30 seconds or so by restoring 2500HP to your targeted ally. Thats insane!!

Situational Items

I usually do not need it, but if you are having a hard time staying away from those annoying stunning champions it can be a life-saver. Especially against a fed eve that will probably always hunt for you.

Short Explanation
Your damage ability which happens to be super awesome and powerful, a bomb will detonate after 4 second hurting enemies around, if another bomb is placed on top of it, the previous one explodes immediately
Decreases all your ability cooldown by 10 seconds, allow you to place 2 bombs at once and late game spam the Time Warp ability as many times as you want
Either Speeds you/an ally up greatly for a few seconds or slows down an enemy, this ability will turn you into a terrible pest for the enemy team
What makes Zilean a great support, hourglasses spin around you/an ally for 10 seconds, if this champion's health gets to 0, he will get back up with hp.

Here are the combos you need to master if you want to become a great Zilean, I even took the effort of making up names for them, I hope you appreciate it!


Early Game
Protip? Math is your friend. Look closely at the enemy's health bar when hurt by a bomb early game to know when your combos will guarantee you a kill. Also count how much mana you will need to perform these combos so that you don't get interrupted because you're oom!

Best Combo
( Optional)__ __ __ __
The range for Ignite is lower than the one for your time bombs, placing it first will makes sure you have the time to place those 2 bombs!

Huho they're tower diving and you're alone!
__ __ __ __
Use this especially if one of the tower divers has a stun, don't forget that you are squishy and they could kill you even before you have the occasion to use your ult. Ulting yourself first will discourage them from staying for too long.


Late Game
Late game, you can just spam your abilities like crazy, allowing you to do these incredibly annoying moves that simply cracks me up.

Light speed
__ __REPEAT
This allows you to have a constant 575 movement speed, you can evade most champions with it and can run around the entire map in a heartbeat

The rage-quitter
__ __ __ __ __REPEAT
Some champions can counter this, many cannot, this will perma-slow your victim, allowing your team to catch up to you if they can, else, you will simply get to do insane amount of damage while they're absolutely powerless to stop you.

Chrono Shift is one hell of a great ability since it starves the other team of kills and can make you have an ace without even losing a single teammate in battle. Although, you will need great reflexes and be able to make choices rapidly in the middle of a battle to determine who should have your ult.

Chrono Shift is NOT something that should be used just to allow your ally to flee safely. The goal of your ult is to keep your ally up in order to immediately bring him back into battle without having to heal back to the base. It doesn't mean that it can't be used this way, it's just not using it at its full potential.

It's hard to pick a rule of thumb here. Most people will say to keep your ult for your carry, which is true. However, you also don't want to let everyone else die around you while you keep your precious ult just for him.
Here are the rules/reflexions I personally follow.

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-Does this champion has an oracle? Priority+

-Who's your team's carry or who seems to be the most useful to your team? Priority++

-Who seems to be in mortal danger and is nearly out of health?
___Is your carry in mortal danger as well? (no)
______Is this player a (stupid) feeder? (no)
_________Priority+++

-Does this champion has a Guardian Angel? Priority-

-Can a Time-Warp probably save him? (yes)
___Will we be in trouble without him in the gank? (no)
_____Time-warp his *** Priority--

-Which champion has a bad/terrible Kill/Death ratio? Priority---

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When should you ult?
As late as possible but still early enough to make sure your teammate won't risk of suddenly getting killed.

Your ult lasts 10 seconds, if you do it too early, the other team will simply wait until its gone before killing your ally. However, if you do it late enough, tney won't have the time to realise what's happening before killing him, allowing him to come back to keep ganking.

Ult + Scream
You will face-palm often when you play Zilean because some of your teammates will run away even with your ult. Which is why you should use this very useful combo.

__ULT!! *Champion name* I ULTED YOU DON'T RUN

Get used to it, it happens All-The-Time. They really need to make something neat like Nocturne's ult where your entire screen would become yellow to indicate that you are under Zilean's protection instead of those tiny Hourglasses!

One last thing, the Ally icons with hp to the left of your screen are your friends. When the battle becomes fierce and filled with annoying particles and special effects, make sure you keep a good eye on all of your Ally's hp. Also its safer to use the quick short keys F1-F5, it prevents you from mis-clicking. Be careful however, they tend to be glitchy sometime.

Early Game

Grab your items and go to your lane its time to have some fun, stay at the turret and do not dare to approach the bushes unless your ally comes in front of you, you are extremely squishy so do not be careless.
Level 1-2, only use your on minions, your bombs are too weak to harass well yet and you would only end up wasting your mana, so focus on leveling up for now. *(protip)* You can however hit 2 birds with 1 stone by bombing one of your Melee minions juuuust as the enemy champion is about to kill it, making it explode in his face, damaging enemy minions around and harassing without spending additional mana.
If there is a jungler, just target the champions instead and last hit the minions to stay close to your tower. However, some people might deliberately go in the middle of their minions to make you push. If it is the case, try to use your bombs only if they wont be able to reach them. Besides that, there isn't much you can do in that case.
Level 3+, You can now start harassing, since you were not bombing the enemy champions until now, it usually has the psychological effect of making them not very careful around you. Excellent, it should allow you to place a few bombs over their heads before they realize you are to be taken seriously. At this point, you however need to limit your use of since you are still very low on mana. Do your best to keep a mana level at least above 250 since it allows you to use your powerful combo if you absolutely need it for a kill or to counter an attack. A low mana Zilean is a useless Zilean.

Mid Game

If everything is going well you should have at least 1 or 2 kills and have purchased your and . If you can, grab the golem's buff which will greatly decrease your ult's cooldown and allow you to spam your and at will. At this point you want to always keep an eye on your teamates' movement, if a gank is preparing itself or if an ally is under attack, zoom as fast as you can to keep yourself ready with your ult. Always stay behind your team and make sure you cannot be reached easily. A good Zilean almost never dies and doesnt want to die to use his ult unless he's doing a noble sacrifice for the team. If you *do* get to use it and your team is competent, afford to throw 2 more bomb on the enemy team before zooming back on a lane, drop 1 bomb on a melee minion, another on a caster minion, zoom to another lane, repeat! Zilean is mostly useful for his ult so keep yourself busy when its under a cooldown!

Late Game

Nothing much more to add by then you should have your and should be starting to build your cooldown reduction while still going for the Golem buff to keep your Cooldowns near that precious 40%. By then you should have around 2500 mana minimum and you can spam your skills as many times as you want. Just enjoy pretending to be speedy Gonzales, help your team, spam the ability all the time to get your ult as soon as possible. If you do everything well, the other team will HATE you while yours is busy worshiping you.

I almost always harass him first, his invisible ability makes him be careless, especially during free twitch weeks

Dodge her stuns and its a piece of cake, she is also one of the champions I go for first

What? Against Heim? Yup, your normal attack range is actually higher than his turrets, making you a terrible annoyance for him, simply attack his turrets and if he gets close to stop you, drop a bomb or 2 on him to keep him away. By doing that, You can even dominate the mid against him. A heimer without turrets is very vulnerable

Your ult obviously counter his and he's usually also vulnerable to your bombs, dodge his toxic attacks and his slow and he won't be a problem against you

Pretty much the same for anyone as long as she's not fed, place a ward without her knowing, when she approached you slow her down, Ignite+double bomb = +1 kill, she usually has a weak resistance against the time bombs

Here are the champions you should be afraid of, no specific orders, the reasons why are in red.

The hell? Isn't she a easy win? Yes, when she is not fed. When she is fed however, she is just an invisible guillotine for Zilean and is one of the only champions that actually FORCES you to include more armor into your build since you cannot just stay away from something you cannot see. Her stun paralyzes you long enough to allow her to crush you. Even if you get oracle, her movement speed will let her get to you before you're able to do anything. If this situation occurs, say hello to paranoia and a constant state of fear for the rest of the game.
See all that red? THATS how scary she can be.

I didn't even need to think to come up with this one. His ult does extra damage depending on how much AP you got and he can stun you. Since you want to get as much mana and bonus AP as possible, he's very dangerous. Stay away from him!

Her ult allows her to dash toward you faster than you can blink, she is extremely deadly for you

Another one that can rape you super quickly, his ult immobilizes you completely and prevents you from using your Chrono shift, I hate that guy.

That god dam Sion.. His shield makes your bombs practically useless and if he has flash, he will use it to surprise you, stun you and kill you. As simple as that.

He cant be slowed easily since his sword spin makes him lunge toward you even if he's under your timewarp spell, not forgetting to mention his ult which prevents him from dying ugh! Not to forget his slow that can't even be countered by your time-warp

The only reason its here is that most of the time he quickly becomes completely immuned to your bombs, don't expect very impressive result when against him

Another one with lots of burst damage, never get too close to that one since she can teleport forward and almost completely drain your hp in a second with her burst spells

I debated for a while if I should add this champion or not here. I decided to put him here because you simply can't do much against him, especially in a 1v1. His range for his lunge attack is the same as your bomb and he can throw you in the air to prevent you from escaping. Even when that happen, its usually not deadly if he's alone, but still dangerous.

Why? Having 1 or more of these champions can make it really hard to use your ult appropriately and I found out about this the hard way.

Anivia
In the middle of the battle, your reflexes might make you ult her even if she has her passive. Annoying when that happens!

Shen
His ult creates a very powerful shield and you both will have the reflex to help a dying champion, if Shen's ult prevent that champion from dying, You just wasted yours! Very frustrating!

Kayle
His ult makes any champion invulnerable for several seconds, completely canceling your ult if you both use it at the same time.

The problem is that, its not like you can coordinate yourself to use your ults alternatively, in the middle of the battle, you don't have time to chat, You just see an ally's health going down quickly you will all have the reflex to do your best to save that person. So yes, having 1 one of those champions isn't too bad, but 2 is terrible.

I'll just let these scores speak for themselves, games usually are like this with a good team =)
..And sometime your team can drop the balls, you still have something to be proud about though

If I get suggestions/critics, I will put my answers there. Don't forget to leave a comment and +Vote +Rep If you like this build =)

Last update: Super long time ago.