Aatrox, the Darkin Blade

Flying, first strike, lifelink, vigilance
This spell costs {1} less to cast for each counter on The Blood Well.
When Aatrox, the Darkin Blade deals combat damage to a player, put one counter on The Blood Well.
Once each turn, whenever a creature dies, if The Blood Well has 5 or more counters on it, Aatrox deals 1 damage to all creatures you don't control. If a creature dies this way, repeat this process. When you can't, transform Aatrox.