Penregon Besieged 

Enchantment
At the beginning of your end step, choose a creature with the least toughness among creatures your opponents control. It perpetually gets -1/-1. When your opponents control no creatures, sacrifice Penregon Besieged.
Legality
standardNOT LEGALpioneerNOT LEGALmodernNOT LEGALlegacyNOT LEGALvintageNOT LEGALcommanderNOT LEGAL