Plague Swimmer (5e Creature)
From D&D Wiki
Large aberration, chaotic evil
Saving Throws Con +6
Amphibious. The swimmer can breathe air and water.
Fear of Fire. If the swimmer takes fire damage, it has disadvantage on attack rolls and ability checks until the end of its next turn.
Multiattack. The swimmer makes three attacks: one with its bite, and two with its claws.
Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) piercing damage. If the target is a creature, it must succeed on a DC 14 Constitution saving throw or contract a disease. Until the disease is cured, the target can't regain hit points except by magical means, and the target's hit point maximum decreases by 7 (2d6) every 24 hours. If the target's hit point maximum drops to 0 as a result of this disease, the target dies.
Claw. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 8 (2d4 + 3) slashing damage.
Vile creatures that lurk in disease-infested bogs and sewers, plague swimmers prefer to dwell in waters touched by disease, though their repulsive presence quickly turns even the purest water into a stagnant mire. They attract creatures that can spread disease, like rats and flies, and they occasionally hunt alongside predators like otyughs that also live in disgusting places. Plague swimmers look somewhat like gigantic toads, though they have the legs and claws of panthers, and a head that resembles that of a shark.