Burningsoul Demon

 is an Inferno Demonlord / Blaze Slayer that has a small chance to be spawned when a player with an active Inferno Demonlord  Quest kills a Blaze.

The chances of spawning one can be increased by the multiplier, which can be increased by the Slayer combat multiplier, Viking's Tear Potions, Wisp Pet and/or a  Wolf Pet.

Trivia

 * has the exact same as a.