Thresh, Chain Warden

Challenger (You may force an opponent's creature to block this creature.)

Level up - When 6 or more creatures have died
while Thresh was under you control, give Thresh
+1/+1 and "The first time Thresh attacks while
levelled up, you may search your hand or library for
a legendary creature. Shuffle if you searched your
library. Then put them onto the battlefield tapped
and attacking."