The Mad Monk was a prisoner of the planeswalker Geyadrone Dihada on Dominaria.[1]
He helped Carth escape the prison and find the Amulet of Ti-Fu and the grimoire to summon Dakkon Blackblade, and so defeat Geyadrone Dihada.
In the end, it was revealed that the monk was in reality a disguised Geyadrone, who in this way put in motion her plot to eventually use the might of Dakkon to kill an Elder Dragon, absorbing its power and subduing the warrior.