This is only my second build so I'm still learning how to make them. Overtime if this gets enough views, votes, and helps people out I will add more to it. As for right now, It's basically just a building guide, not a playing guide.

If you can get a good number of assists or kills early game than you can shy away from armor after your sunfire cape and then build attack damage and attack speed. Then end game once everyone else catches up to damage, you go for a frozen mallet to really put the nail in the coffin on anything they try to do to counter rammus.

A lot of people build "heart of gold" with Rammus, but this build is 15.4k to finish. A heart of gold is good beginning game, but once the Rammus snowball effect kicks in then the heart of gold is useless and a waste of gold.

Beginning game for me is all about supporting the ADC or whoever you are laning with. Not really trying to get a lot of kills, if they come they come, if not that's ok. Because once you finish your sunfire cape, you are pretty much in control of anything that happens on the playing field. That's when you can really attack, move around, gank, and control the tone and speed of the game.

With this build I have solo'd baron, I have took on 3v1, tank turrets. Everything you would love to have in a Rammus.

Pros!

Really strong defensive attack

Invincible in Defensive ball curl

Can solo baron with no lifesteal or healing :)

Really strong ganker

can tank turrets for the team and lose hardly any health

hard to counter bc he does a lot of both AD and AP damage

Cons!

no magic resistance(but his health and attack damage are high enough it doesnt matter)

with all those spikes he's hard to pet :(

without the powerball and a flash he can be a little slow moving around, always undo defensive ball curl when running away

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For runes, I love to build runes that give you everything in the beginning. It feels that it is more benefitial to have that extra umph in the beginning rather than waiting over time for your runes to really give you a kick. I use the health runes for quintesences and for marks because it gives you an extra 108 health in the beginning. When most champions start at around 500-800 health in the beginning, having an extra 108 health really makes rammus a force to be feared right off the bat. Armor, ofcourse to help his passive, which gives you an extra 13 armor and extra 7.5 (or 8) attack damage. And your cooldown reduction, which 6% can honestly come in handy for ANY champion. IMO.

This build I feel is great for Normal matchmaking. I have not tried it in Ranked, but I feel if you buy the same items as jungling you will get the same outcome and be just as beefed up. I start off with Cloth , 3 Health Health Potions and 2 [[Mana
Potion]] , Each time you use Rammus' Powerball it cost 70 mana, you use that skill 4 times before you hit lvl 2 and you're out of mana. So bringing 2 mana potions I believe will help you be able to stick in the lane long enough to maybe snag a couple kills or assists without having to worry about mana issues. The idea is trying to stay in the lane as long as possible at first to try to gain more gold than your lane enemies. Or if you are jungling, than you would want to stay out in the jungle longer than everyone stays in their lanes. For this you would not need any mana potions, because you would have the blue buff :) So I only take health potions when jungling.

Thornmail I've been told to grab health before I grab a thornmail because a thornmail wont work if you don't have enough health to back it up. Honestly to me, the reason I get the thornmail first is because the armor it builds does two jobs with rammus. It builds your armor, and your attack damage. Which I feel in the beginning is a little better than health because no one else except maybe their tank is going to be building health early on. So if you can build armor to counter their damage build and build damage on top of that with your armor, you will be better off.

Sunfire Aegis After you get your thornmail, next would be the Sunfire Cape. I love the sunfire cape because it creates what I like to call the "Triple Effect" Your thornmail, Sunfire cape, and Defensive Ball Curl ALL do damage to enemies around you, or just enemies attacking you. So when you jump in to initiate a battle and you have 3 people attacking you and you alongside 2 of your guys attacking them. You just taunt the ADC and go into defensive ball curl right away and you can take them out of the picture for 1-3 seconds depending on how far it's leveled up, do damage to them, and the other 2 that are around you. Ultimate CC (Crowd Control).

Madred's Bloodrazor Madreds bloodrazores are one of my favorite AD items to buy. It gives attack damage, attack speed, and armor. what more could you ask for? OH yea... not to mention it deals 4% of the enemies health in magic damage... on hit. Which makes the next item sound a little better, because it deals more attack speed! This could also be called the "Quadra Effect!" :P damage from sunfire cape, damage from thornmail, damage from rammus' defensive ball curl, damage from rammus, and damage from bloodrazors. If they have 3000 health, thats 120 damage everytime you hit them... if your attack speed is 1.3 per second, than during a 3 second taunt your doing about 360 damage just from your bloodrazors.

Black Cleaver In a game an enemy Xin Zhao that had over 20 kills goes in ALL chat saying "Rammus you got a black cleaver?" I then scouted the map looking for him, I only had about 9 kills at this time and little over 10 assists, when I found him I immediately taunted him and used my ulty on him. Killing him in about 6-7 seconds. I then said "yea, you like my Black Cleaver?"

After you build the thornmail and sunfire cape, you will be at about 200 armor. That 200 armor gives you 50 attack damage, after you buld the black cleaver you will be over 200 armor and over 100 attack damage. Which will be the same attack damage an underfed ad champion will have. BOSS STATUS! Black Cleaver gives high damage and attack speed, Rammus could use this! All AD champions could use this. It also reduces the enemies armor by 15 for 5 seconds, may not seem like much, but any little bit helps.

Frozen Mallet Last but not least, the frozen mallet. If the game lasts this long, You will see that people are going to be finding rammus rather hard to kill. They are going to try to counter you anyway they can, whether it be by building a force of nature, or a thornmail. Trying to double and triple team you. Whatever they can think of to try to take you out of the equation. Frozen mallet gives you 700 more health and 20 more attack damage. But the passive is what makes it so great. on top of all this, it adds the beneficial "on basic attacks reduces your targets movement speed by 40% for 2.5 seconds". so if you have someone that stuns you and you cant move, when you're done you can hit them, then run a little bit so that your far away to go into your ball and roll away without being too close to hit them and stop it.

Yea, it's nice to try to be a full on AP with rammus, but also understand that Rammus was built to be a tank. When people see Rammus, you want him to strike FEAR in their minds. So you still want him to be tanky. This build only adds 310 Ability Power, but he still has a ton of armor, and a small chunk of Magic Resistance.

The Holy Grail gives him cooldown reduction, along with the runes that's a little over 30% which is great because the biggest burst of ability power Rammus uses is his Ulty. Adding ability power into the mix means Rammus can pretty much take down a tower in the time his ulty is going off. About 8-12 seconds. Plus he's so tanky, he doesn't even need minions :D It's like having a buffed up master yi, yet he can rush into a group of enemies as well :D

Rabadons Deathcap ... nobody needs to explain this item :P 30% more ability power? yes please.

Rylai's Crystal Sceptor - Adds more ability power, plus gives you mana regen (which rammus will need) and more health! AWESOME! :P

I like taking a point in each. Starting with Powerball. Then I grab my Punturing Taunt, then the Defensive Ball Curl. After that (Almost Every Rammus) levels Puncturing Taunt to 5 as fast as possible. Reason for this is because the longer you can taunt your enemy the longer you and your team can hit them without them running away. If you know that the person is not going to run away and stay attacking, then don't even bother until they get a little low health. But in team fights you want to swoop in with your powerball and try to go straight for the ADC (Attack Damage Carry) or whoever is doing the most damage to your team first.

Next thing to level up would ofcourse be your Defensive Ball Curl because the more you level it up the less damage you take and the more damage you return :)

Lastly would be the powerball.

In the beginning farming is slow. As is with almost every other champion in League of Legends.

But once you finish building your sunfire cape, carming becomes a breeze :P just run up to a group of minions, get in defensive ball curl, and watch a group of about 20 minions die in less than 5 seconds.

8/6/2012 - Updated the item purchases, to show the health and mana pots that I usually start with. Then just put in the items I buy in order of which ones I purchase first.

At this moment, this is all I'm going to add to this build unless I get a couple of views and some good votes. :)