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
'Whenever' always hint at the fact that it can happen multiple times. Stuff that only trigger once are usually written as 'when'.
The (somewhat) balancing factor is, that it is unlikely, that a creature with 4 toughness can attack and be blocked multiple times without dieing. Why would an opponent keep creatures back from the first block? It can obviously happen through combat tricks. But I don't see more than one additional attack phase happening very often.