Anzrag, the Quake-Mole - 2RG
Legendary Creature - Mole God (Mythic)
Whenever Anzrag, the Quake-Mole becomes blocked, untap each creature you control. After this combat phase, there is an additional combat phase.
3RRGG: Anzrag must be blocked each combat this turn if able.
His claws tear at the foundations of civilization.
8/4
As long as Anzrag keeps getting blocked, you keep getting more combat phases. There is no hard limit