Shieda Kayn, Shadow Reaper

Shadowstep {1}{b}: Whenever Shieda Kayn, Shadow Reaper deals combat damage to a player, you may pay {1}{b}. If you do, exile him, then return him to the battlefield tapped under your control at the beginning of your next upkeep.

Assassinate {t}{2}{b}: Destroy target tapped creature.

Rhaast’s Corruption: At the beginning of your upkeep, if Shieda Kayn, Shadow Reaper is exiled, put a corruption counter on him. If he has three or more corruption counters, transform him into Rhaas