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.
He will then buff himself with Vigor of Sseratus: Empowered by Sseratus. Attack and casting speed increased by 60%. Damage taken reduced by 30%. Lasts for 2 minutes.
Prophet of Sseratus yells: NOOOOOOOOooooooo.........!
The Prophet will shrink down to regular size, becoming Muddled Prophet of Sseratus, and then attack you.