Whenever Aatrox consumes a portion of his health, he stores it into his Blood Well, which can hold up to 105 - 870 (based on level)health. The Blood Well depletes by 2% per second if Aatrox hasn't dealt or received damage in the last 5 seconds.
Aatrox gains 0.3% - 0.55% (based on level) bonus attack speed for every 1% in his Blood Well, up to a maximum of 30% - 55% (based on level) bonus attack speed.
Upon taking fatal damage, Aatrox enters stasis and drains his Blood Well, healing himself over the next 3 seconds for up to 35% of the Blood Well's maximum capacity, plus its stored health upon activation.
Blood Well is both a self-targeted buff and a passive ability that triggers when Aatrox takes fatal damage.
The maximum health Blood Well can restore is 141.75 - 1174.5 (based on level).
This is equivalent to 135% of the Blood Well's content, but the heal is composed of a base value of 35% of the maximum pool, plus 1% per 1% of its current content.