Beyond the engagements with him at Caldera's Keep, it is unknown where Bismuth comes from.
Bismuth appears to be related in some aspect to Iskar, as she seems to know him, to the point where she angrily demanded him to reveal where 'he' was, speaking of the Cult Leader. His only retort was that instead of worrying about a certain pair of monsters, she should have been gracious and accepted the Cult's offering of power, whatever it might have been.
He is encountered a second time in the Pito Caves, once again serving as a mini boss, but is far more powerful than when the Player last met him. He demands the Hero give up and back down, lest he destroy the Player. Failing to ward them off, a duel begins between the two, resulting in Bismuth's defeat. Bismuth disappears, swearing vengeance and assuring the Player's defeat at the hands of the Cloaked Man.
Here are the stats for Bismuth during the mini boss encounter.
Bismuth will attack, defend, cast Greater Wind, Thunder, Fire, and use Leg Sweep, Beast Slayer, and Armor Break at any time.
At 70% Health he can begin using Mass Wind, and at 50% Health Bismuth begins using Mass Darkness, Radiate, and Greater Heal.
He will periodically comment on the Player's capabilities as they continue to wound him.