The degenerate Hullghasts are a debased form of the far more common Ghilliam. These grotesque mutants exist in the darkest depths of voidships and space hulks, deep in the forgotten corridors and desolate holds where atmosphere, temperature, and radiation all wreak terrible havoc on the body and mind.
Hullghasts are the result of such twisting influences, and these vile creatures long-ago gave up all connection to the Human race.
Predators in the truest sense of the word, Hullghasts prowl the lower decks of derelict voidships and space hulks seeking meat in whatever form they can find it. They are aggressive and violent and seem willing to attack openly when desperate for food.
The Hullghast mutation allows the creature to survive in the most hostile surroundings known to Humanity. They are immune to most environmental hazards such as poisons, mild doses of radiation, pollutants, intense gravity fluctuations, airborne toxins, and almost anything else that could be encountered in the depths of any voidship.
Most alarming of all, what does not kill them does indeed made them stronger. If a given Hullghast survives a form of poison, be it toxic or venomous, they become immune to said-poison's effects indefinitely.
- Deathwatch: Ark of Lost Souls (RPG), pg. 118