Bajilim

Abyss Dweller. The bajilim breathes in saltwater, and is unharmed by the pressure of deep ocean. If exposed to low-pressure waters, it takes 4 (1d6) bludgeoning damage for each turn it lingers.

False Appearance. For as long as it holds still, the bajilim appears to be an ordinary flailsponge. If investigated (DC 17), it can be recognized by its symmetrical shape.

Sting. Melee Weapon Attack: +4 to hit, reach 5 ft, one target. Hit: 10 (2d6 + 1) poison damage. Targets must make a Constitution saving throw (DC 14) or become poisoned, suffering the effects over the course of 1d6 days.

Each day, a new symptom manifests;

• Day 1: Blood loses oxygen, turning the skin pale and causing shortness of breath
• Day 2: Rod cells deteriorate, causing partial blindness
• Day 3: Hallucinations in peripheral vision
• Day 4: Catatonic schizophrenia
• Day 5: Rapid muscle/fat loss
• Day 6: Leprosy of the fingers, legs and eyes