Ob Nixilis, Unshackled 
Legendary Creature — Demon
Flying, trample Whenever an opponent searches their library, that player sacrifices a creature of their choice and loses 10 life. Whenever another creature dies, put a +1/+1 counter on Ob Nixilis.
Legality
standardNOT LEGALpioneerLEGALmodernLEGALlegacyLEGALvintageLEGALcommanderLEGAL