A pet class created when an asshole pet under a soul protection spell dies and returns demon-fused.
The dungeon's entry for Bianca Del Ciao explains the Hellspawn Familiar: it occurs when a regular, genuinely mean-spirited pet is encased in a soul protection spell yet dies anyway. The soul becomes hellbound and tainted, then is shoved back into the resurrected body, rebirthing the creature with its natural form merged with that of a demon. The system illustrated the concept at length with Susan, the Pembroke Welsh Corgi given to the future Queen Elizabeth II, who bit police officers and the royal clock winder.
Such a familiar physically grows with every level-up, stopping only at level 50, and grows wings. It is angry, formidable, scary as shit, and one of the most powerful and badass pets available within Dungeon Crawler World.
Pet classification governing Bianca Del Ciao's demonic form and growth.
Also appears in books you've already read — spoiler-safe to their endings: