Whenever Stubborn Burrowfiend becomes saddled for the first time each turn, mill two cards, then Stubborn Burrowfiend gets +X/+X until end of turn, where X is the number of creature cards in your graveyard.
Saddle 2 (Tap any number of other creatures yo control with total power 2 or more: This Mount becomes saddled until end of turn. Saddle only as a sorcery.)