Colossal Catfish (5e Creature)
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Gargantuan beast, unaligned
Hold Breath. While out of water, the catfish can hold its breath for 30 minutes.
Slippery. The catfish has advantage on ability checks and saving throws made to escape a grapple.
Water Breathing. The catfish can breathe only underwater.
Bite. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 17 (2d10 + 6) piercing damage, and the target is grappled (escape DC 18). Until this grapple ends, the target is restrained, and the catfish can't bite another target.
Swallow. The catfish makes one bite attack against a Medium or smaller target it is grappling. If the attack hits, the target is swallowed, and the grapple ends. The swallowed target is blinded and restrained, it has total cover against attacks and other effects outside the catfish, and it takes 10 (3d6) acid damage at the start of each of the catfish's turns. The catfish can have only one target swallowed at a time.
Colossal catfish are uncommon, slow-moving filter feeders, dwelling at the surface of massive rivers and deep lakes. Some water dwelling races use them as effective long-haul mounts, even running shops on their backs. Despite their dull appearances, colossal catfish are rather intelligent, capable of understanding some basic commands.