Mulahey is a cleric in the employment of Tazok, and the boss of chapter two in the storyline. He is found at the lowest level of the Nashkel Mines. He has somehow convinced the kobolds who work for him that he is one of their deities.
He will summon hordes of kobolds and skeletons; focus on attacking him or try and group them all together and use AoE spells.
He can either be killed and the party can retrieve his letters of correspondence from the chest or, when he surrenders, be forced to reveal what he knows.