I am Trephneor and this is my first guide on Mobafire. I hope you like it, feel free to suggest stuff or contribute to the guide, I'm open towards improving!

Rek'Sai is the perfect balance between the so called "faceroll" Devourer junglers and the ones that require practice and skill, like the one and only Lee Sin. Don't take me wrong, I know that there is a difference between a good Warwick and a bad one. Every champion needs to be mastered, some are just easier to do so.

But we aren't talking about normal Rek'Sai here, but the one and only AP Rek'Sai tornado spamming AoE demon. With full build, it'll do over 1k damage along with the 180 base damage, with a little less than 5 second CD (with 40% CDR).
Read on, if you are interested. Leave if you are the tryhard kind.

Pros

+ AoE damage + Huge kiting potentional + Risk free harass + OP Lategame

Going into underground with your Burrow provides you extra movement speed, while you are able to shoot your Prey Seeker constantly backwards. After buying Rylai's Crystal Scepter it even slows them down, since this items passive is an ability slow. In Rek'Sais case that means AoE slow.

Cons

- Glass Cannon - Tricky to handle the vision in Burrow - Can't use most of Rek'Sais kit in teamfights - Weak early

You have to be smart, if you want to play AP Rek'Sai. Because of Rek'Sais kit, you'll still be able to clear jungle pretty easily after getting Ranger's Trailblazer, but you'll have to watch yourself carefully in a teamfight. Anyone closing on you can kill you easily.

You are a glass cannon. Never EVER forge that. You should be mostly out of range from the enemy team while constantly harassing them with your Prey Seeker. It is an extremely long range, low-cooldown AoE Javelin Toss. Don't forget that your Tremor Sense can help you harassing enemies that are in the Fog of War.

Try to avoid combat by digging Tunnels beforehand, and you can also use it as an escape. Watch for it's cooldown though. Make sure you have it up, when it's needed!

Let's take a look at the runes
We are taking pretty basic AP carry runes, with armor. What is interesting there are the CDR runes. Shooting that Prey Seeker, or how I like to call it Torpedo of Doom.
You can of course swap the CDR with some more scaling AP glyphs.

Ranger's Trailblazer - Magus

Your first item must be Ranger's Trailblazer, otherwise you will have big problems clearing your jungle. The AoE smite along with the stun gives you enough speed in case Rek'Sais epic kit wouldn't be enough.

This is your core item. It synergizes extremely well with your Prey Seeker, slowing the enemy in AoE while dealing huge AP damage. Don't forget that it deals 60 / 90 / 120 / 150 / 180 (+ 100% AP) damage! Also it provides you with some sustian due to the 400HP it gives you.

You always wanted Nidalees spear of death to be a slowing AoE DoT projectile, right? Well, with both Liandry's Torment and Rylai's Crystal Scepter in your pocket, that dream came true. You are already dealing quite some damage and penetrate their defenses. Liandry's Torment also gives you HP. Even more sustain.

You may start asking at this point, could it be better? YES! Buying your Void Staff next clears the rest of their resistances and truly starts the harass dream,

All you need now is even more damage. Because you are playing a glass cannon, there is no need for defenses. The sustain you get from your Rylai's Crystal Scepter and Liandry's Torment is more then enough for you to forget worrying about the jungle monsters.

Last, but not least, this very special little blue orb. You might be laughing at this choice. But if you aren't a challenger level player, processing information faster then Wall-E, then you might be getting confused from all the steps you see with the Tremor Sense if several enemies are in range. Or you may just want to check, before you go in with your Void Rush. Also good for checking Baron Nashor or Dragon.

Shoes

For super speed torpedo purposes. Or as would Jinx say: Shoot faster!

If you aren't confident enough to have no defenses, this might be a good choice to pick up if enemy has a strong AP carry.

Enemy AD carry too stronk? You die in jungle for some awkward reason? Buy Ninja Tabi!

If the rare occasion occurs, that they somehow managed to stack enough magic resist against you, then this will be the killing blow.

> > >

Maxing your Queen's Wrath/ Prey Seeker is a well explained and pretty obvious thing at this point of the guide I think. It is your core skill and the most important to max out. It is also good for jungle clearing if not in Burrow. Taking Furious Bite/ Tunnel makes you more mobile, because of your epic underground labyrinth and helps you clear jungle faster Unburrowed.

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Fury of the Xer'Sai: Your passive is why Rek'Sai is between Warwick and Lee Sin sustain wise in the new jungle. When going in to Burrow you regenerate almost back to full HP usually, since the rage bar fills up with very few autoattacks/spells.

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Queen's Wrath / Prey Seeker: It is your torpedo. Your life. Your purpose with AP Rek'Sai. Constantly use it to harass or kite. It's cooldown is 11 / 10 / 9 / 8 / 7, which can be reduced to 5 seconds with Blue Buff and Ionian Boots of Lucidity.

• Use the information you get from Tremor Sense to harass into the Fog of War
• You can use this ability to kite, with the increased movement speed you get from your Burrowed form. After getting Rylai's Crystal Scepter, this becomes even more easier to do.
• Use it to check buffs/baron/dragon before approaching, because it provides you vision on the enemies struck.

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Burrow / Unburrow: Provides you mobility with the +15 Movement Speed you get from Burrowing, helps in ganks and jungle clearing because of the knock up. The Tremor Sense, which is the passive of Burrow makes it easy to spot enemies in the Fog of War.

• Use the knock up constantly while clearing the jungle.
• The purple circle around enemies that have been knocked up already shows you the time before you can knock them up again.
• Develop the habit of using Burrowed form all the time. You will be faster, you can shoot torpedo's and sense everyone.
• Watch out for campers! You cannot notice anyone who is not moving! Use the Farsight Orb if you are unsure what's happening. Or Unburrow.

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Furious Bite / Tunnel: Furious Bite adds well to your burst damage ganks and helps clearing the jungle. Tunnel grants you mobility and makes your ganking hell of a lot easier. You can check back later on your cute tunnels with your Void Rush.

• Practice which walls can you go through with Tunnel. You will be surprised by it's range.
• Use tunnels constantly, even when you aren't in danger or in the process of ganking. It's just cool and you can enter them later on without any cooldown on entering the next tunnel. Super fast movement.
• If you are really advanced, you can keep your eyes on the minimap. If a tunnel is destroyed, an enemy is there.

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Void Rush: Quick repositioning, cool global map sound effect, what else do you need? Oh, right: a great passive!

• The channel time is only 1.5 seconds. If you get far enough from the enemy, feel free to use it as an escape. Rito wants you to believe it's not meant for that, but it works.
• Your destination is known by the enemy if they have vision on that tunnel. You also make a global screaming sound. Plan your appearances accordingly.
• The passive helps you clear the jungle faster.

Here are some videos that can help you understanding the playstyle of AP Rek'Sai.

Siv HD - AP Rek'sai

Play Video
I will add more videos as they come out.

2014.12.22: Added Rek'Sai mid build
2014.12.17: Added Rek'Sai top build
2014.12.15-16: Minor fixes(spelling mistakes, some design addition)
2014.12.14: Updated with the actual guide, nice layout too :3
2014.12.13: added in basic items