Slay the Muddled Prophet of Sseratus and then return to Hexxer Ubungo at The Argent Stand.
There. I've modified the mojo. Now when the prophet drinks it, not only will he lose some of his power, his mind will also be addled by the effects of the fungus.
At least that's the theory.
As for how to get him to drink it, might I suggest you simply throw it at him? No doubt he'll catch and drink it given the arrogance that he's displayed at the altar.
Unless you can kill him, [class], your Argent Crusade is about to come to a terrible end.