The answer is pretty simple: His spells.

His E has an 0.6 ap scale which is pretty good. And unlike the most multihit-skillshots, it's damage doesn't decay after minion or champion hit. This means you can fire this trough a minion wave and hit champions after it and everybody will get the full damage from it or use it in a teamfight to get the full damage on as many enemies as possible. Together with the pretty low cooldown when maxed, the fact that it's one of the easiest skillshots to land in the game and the missing mana costs, you can spam this spell easy to poke your enemy down.

And then there is his R... it actually scales 1.0 ap. This together with the high base damage on it and you deal insane AoE damage, and the cooldown in it is pretty low too if leveled.

So that's it on the ap side? Yeah, and a 1.6 ap scale doesn't seem that much, i know, but believe me, it is enough since your E is so easy to land (good for luden's) with that much range and you got to deal the full damage potential of your ulti this way cause normally you woudn't go in and then use ult but rather use it before (at least if you are not going fulltank Aatrox).

What you might want to do is increasing your ap scaling with items like Lich Bane or Nashors (since Aatrox is normally an AA-depending champion). Don't do it! No one loves these items more than I do, but I will explain later why that isn't such a good idea.

(The perfect moment to pick him is if your team is full ad except you. The enemies will stack armor and get poked and bursted even easier.)

Pros:
+ Good laning phase
+ Snowballs hard
+ Can splitpush good
+ Mage without mana
+ Good poke
+ Good AoE burst

Cons:
- Team might get mad so ask them first
- Has to jump in to get full burst off
- No heal, even tho spells cost hp
- Not as much single target damage than other assassins in late

One of the best passives in the game. This + zhonias and you can engage even tho you are damn squishy and get enough damage off so your team can easy win that fight until you are back and even if not, you should have another E up again to finish it yourself. It also gives you first blood pretty often.

Really nice to engage fight with E > Q > R if in range of E or Q > R > move a step so they are in a line > E if not, but don't feel the need to use it everytime you fight someone. You don't need it for the damage, more as an escape or to follow. Also, don't farm with it. It might be annoying to have every minion survive with a few HP after your E, but you give so much mobility up if you do it. Max this secend just for the cooldown reduce.

The core spell of ad Aatrox, not that useful for ap Aatrox tho. Put a point in it at level 2 or 3 for the little heal in lane, but that's it. You don't want to be in aa-range, throwing out your dps if you are that squishy anyway, so no need to build anything around this spell or invest too many points in it.

This spell is just awesome. I have no idea where all the damage comes from but it deals rly good damage. Try to use it to get some lasthits without having to go too close to the enemy and hit the enemy champion with it at the same time, if you manage to do this you have already won the lane. Level this first ofc for the reduced cooldown and damage increase. Btw, this can be strange to hit if you are too close to the enemie so take a little step away and then use it, if you have the time for it.

I never thought I would deal that much damage with this spell when I first saw it. In late game, you just deal over 1000 damage with it. Instant, AoE and damn easy to land. Engage in the right moment with Q and R on as many enemies as possible and there is no way your team will loose this fight, especially cause you most likely already poked them down with your supercheap 1000 range E + Luden's.

The reason I rather pick Boots + 4 pots is cause I need the pots so I can poke with my E. While Doran's ring gives a good amount of mana for the spells normally, it is just wasted in Aatrox hands. Sure the Hp and Ap is nice, but I'm just not a fan of wasted stats and you can stay way longer on lane with 4 pots than with +60 hp and 2 pots.

Makes your poke even better. Also rly nice for waveclear since it will focus the 3 melee minions and the cannon one while the ranged ones die just from the E alone. Once you have this, you can still poke them through minions, but it would be better if you hit them directly or at least only hit a minion close enough to them so the luden's passive hits them too.

Yeah, gives you ap. Don't know what else to say about it, just buy it!

There we go, the one legendary item which gives every not ap champ an ap scale. You can increase your burst with it, especially on lane, BUT it just doesn't fit. If you poke, you want to get close, throw your E out and avoid a fight. No need to explain why you can't throw out an aa to get the passive off. And if you engage, you could be aa once for the passive, but believe me, you don't want to. You jump in, activate your ult, try to get a good E off, there is just no time for the aa except after your combo when you already destroyed them, and then a chase of the survivng shoudn't be too hard with E.
So you can use it to increase your damage a bit in teamfights, but I tried it pretty often and don't think it is worth. Use it rather on champions who can use it easy to poke or have an aa or on-hit effect in their combo anyway.

No! Never go this item. You have no reason to do so. Everytime I explain ppl the strengh of ap Aatrox, their first reaction is "But shoudn't you pick up Nashors?". I know why many think this, Aatrox is just a very aa/as depending champion normally, but you aren't playing that champion anymore. You are playing a champ who pokes on a distance and jumps in to burst all at once when they are low enough. You have no time to stand in there and get your small dps with Nashors off, expecially since your W scales with ad, so even if you manage to get in there and aa them a while without dieing, you are just dealing as much damage as any other melee-champ with nashors.

Gives you rly much ap and an incredible annoying active. You are just rly hard to get killed if you use this after your burst so they have to fight your team first and even if they manage to spare some damage for you when it ends, you might still have your passive.

Nice synergy with ulti and the mr is not bad. Since you will be in the middle of the enemies in teamfights, it is also nice for every magic damage dealer in your team if you buy this.

You are really squishy if you take this as last item, but your damage is insane. A nice option if you are ahead and have to win the next few teamfights to finish the game.

A good mix between Hp and Ap on the last item and the slow is not bad, even tho it is just 15% and you already have a 40% on your E.

More of a fun iteam if you want to be unkillable with zhonias, passive and this one. If you have to carry them, I woudn't go for this, simply because it doesn't give you any more survivability since you already have resistance and even more doesn't make you much tankier if you only have your base Hp. And yes, you respawn with at least another E, but that doesn't help you if your team dies while you are busy getting out of zhonias, respawning from your passive and then respawning from zhonias.

Ik it is most of the times better to go for a Hp/resistence item, but you already have some resistence so this might be a good option not to die as fast.

Makes you pretty tanky and you can use it right after your engage so their entire team is slowed.

Blocking the next ability is just great if you want to get your burst combo off more easily. Woudn't pick this item tho unless most of their damage is magic.

Boots with magic pen. Should be pretty obvious.

Yeah, cdr for E is nice and also for Q so you have more mobility, but I's rather pick magic pen. If you want to fo for these, you should at least get Void Staff as last item or over Abyssal Scpeter imo.

You need it so often to get the last bit of damage off. I can't think of a good replacement on him.

You guys know why we always pick flash.

Nah, you should concentrate more on dealing damage than on your own hp. Use zhonias and passive to avoid getting killed.

You have a slow on E and a knockup on Q. Concentrate on dealing as much damage as possible in this time instead of trying to reduce the enemies damage.

The only spell you could use instead of ignite. I still prefer ignite tho since you just need pots to stay in lane, midlane doesn't take too long to walk back to lane and you can help mid/bot/dragon without teleport if you are playing mid.

Not worth. Sure you can chase good with it, but thanks to Q and E that shoudn't be a problem most of the time. And to escpae, flash + E + Q is just better.

Actually, he is a decent champion. I'm not going to be all like "here, build this and win every game instant" all over you guys cause that is complete ********, you still have to learn him to play him well, but I think this way he can be a very viable mage/assassin.

But should you play him in ranked? ONLY if you team is fine with it. I don't say that cause I think he isn't as good as a normal mage (he might be one of the best midgame champs) but because they will often think you just want to troll and get mad way easier if you actually loose lane (and you will seomtimes. You enemy laner will be just a bit better sometimes or you will have a bad day, there is no guide that can protect you from that) and that will be a rly bad game for you, the guy who got mad and the 3 other guys who just wanted to have a nice game. So ask them first and at least 3 of them should say that they are ok with it.