Riku, Pawn of the Darkness

First strike
Whenever you or a creature you control takes damage, put on dark counter on Riku, Pawn of the Darkness. When Riku, Pawn of the Darkness has five or more dark counters, transform him.
"Riku gained immense power through the forces of darkness. But ordinary humans are no match for the darkness, and it is gradually consuming his heart." -Jiminy's Journal