Kayn, the Shadow Reaper

Haste
{u}{u} Surveil 2. If you surveilled 5 times this turn, sacrifice Kayn and return him to the battlefield transformed as Shadow Assassin.
{b}{b} Sacrifice a creature. If 3 creatures left the battlefield this turn, sacrifice Kayn and return
him to the battlefield transformed as Rhaast.
"The bigger they are..." - Kayn. "The louder they
scream." - Rhaast.