@Five Second Pose and I have spent quite a bit of time in IRC discussing how to build Aatrox and what he provides to teams. Ray insists on building him as a spell-heavy bruiser/disruptor, while I think he is best played as an ADC, but we agree that both are viable. I've spent way too much time thinking about this so I thought I'd make a thread about it.
My summary of his kit:
Blood Well: Places him into a stasis when he dies. When he exits stasis he recovers HP based on how much HP is in the Blood Well. HP is stored in Blood Well whenever he casts a spell with an HP cost, and all of that HP cost is stored in Blood Well. He gains AS for every 2% max HP in Blood Well.
This is the core of his viability as an AD caster. The more spells he casts, the more HP he recovers when his passive activates. The passive AS buff is nice, but it doesn't contribute much to disruptor/AD caster gameplay.
Dark Flight: This is a lot like Elastic Slingshot. Aatrox jumps 650 range (Caitlyn AA range), deals AoE damage, and does an AoE knock up. It has a 60% total AD ratio and costs 10% of his current health.
The big thing here is the 10% current HP cost--if he is building tanky with HP, that's a lot of HP stored in Blood Well. Combined with CDR, which is Ray's theory, Aatrox builds up Blood Well really fast and has immense presence in fights. It's a nice initiate/gap close/escape too.
Blood Thirst/Blood Price: His W is a toggle; Blood Thirst heals him every 3rd AA. Blood Price deals bonus damage every third AA. Both on-hit effects scale off AD.
Aatrox's W is 80% of the reason I think he is a better ADC than he is a bruiser. I'll explain that later.
Blades of Torment: Pretty long ranged (1000 range, same as Mystic Shot) magic damage slow that scales off AD. Standard CC fare.
I think Aatrox's E makes him extremely strong in lane, not because of the slow or the range on it, but the fact that it is magic damage. People will be building things like cloth/5 potions and generally armor against him in lane, but because Blades of Torment is magic damage, ~70% of its damage will go unmitigated in lane and Aatrox will win trades--and it scales off AD. Its cooldown is relatively short for it being a slow (12 seconds at rank one to 8 seconds at rank five).
Massacre: Does AoE magic damage around Aatrox and increases his range to 325. Gives 40/50/60% AS buff.
Honestly this ult is fucking OP and it will be nerfed. If Aatrox hits 6 either before you or at the same time as you, he will all-in you no matter how you itemized, because his ult does magic damage, and like his E, a lot of its damage will go unmitigated, meaning he has very strong front-loaded burst plus a slow, plus good sustained DPS because of the AS buff and increased range. Even if Q -> R -> E doesn't kill you, the faster W procs will. Once Aatrox hits level 6 you cannot 1v1 him. This doesn't even take his passive into account.
Why I think ADC Aatrox is better than bruiser Aatrox:
Bruiser Aatrox works like bruiser Vlad, in that he builds a lot of HP and a lot of CDR to spam his spells and generally be annoying. We emphasized HP, CDR, and Spirit Visage on Aatrox for the same reason they are good items on bruiser Vlad--those items let Vlad constantly Transfuse, which adds high sustain to the tankiness that comes with building HP. Similarly, HP makes Aatrox tanky, which allows him to be less cautious with Dark Flight; CDR lets him spam spells, which means more CC and more HP into Blood Well; SV means Aatrox recovers more HP with Blood Thirst, and he will come out of Blood Well with higher HP as well.
However, I think the sustain that Aatrox gets from HP/CDR/SV is mirrored, if not outdone, by AS and lifesteal, meaning a damage heavy build, focused particularly around one item: either BT or BotRK, depending on what Aatrox wants to do in a game. My reasons:
1. Blood Thirst, Aatrox's heal, scales off AD, and procs every 3rd autoattack, meaning it effectively scales off AS. This means that AD (Bloodthirster), AS (Phantom Dancer), or both (BotRK), give Aatrox sustain on top of the lifesteal that he will build. In short, Blood Thirst lets Spirit Visage be replaced by AS/AD items. Yes, this sacrifices health, magic resistance, and cooldown reduction, but I think the sustain that an ADC build gives Aatrox is comparable to the tankiness that Spirit Visage and a bruiser build would give him.
2. Blood Price has a 100% total AD ratio and effectively scales off AS as well (procs every 3rd autoattack). Bloodthirster comes to mind as the single best item that complements Blood Price. It gives 100 AD at full stacks, which plays into Blood Price's scaling. It gives 18% lifesteal, which mitigates the HP cost of Blood Price's proc. Combined with Phantom Dancer or Statikk Shiv and Berserker's Greaves, Aatrox will be procing Blood Price very often with no net HP loss.
3. Blood Price costs HP, which is put into Blood Well, which increases his attack speed. Blood Price's HP cost scales with Aatrox's AD, meaning more HP into Blood Well, meaning more bonus AS. Building ADC on Aatrox will actually cause his passive to increase his DPS.
4. Blood Well's HP recovery does not scale with anything except level and Spirit Visage. At level 18, Blood Well restores at most 840 HP, 1008 with SV. It doesn't matter how much HP Aatrox actually has, he's still only going to recover 840 or 1008 HP after coming out of stasis. Aatrox will still be able to abuse his passive with a damage build, albeit not as effectively as with a bruiser build.
I've actually tired myself out in writing this. Maybe I'll add more later or in later discussion, but my point is that his kit suits an ADC build.
Builds I've proposed:
And some bruiser builds Ray and I have conjured:
Feel free to discuss.