Mosasaurus (5e Creature)
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Gargantuan beast (marine reptile), unaligned
Brute. A melee weapon deals one extra die of its damage when the mosasaur hits with it (included in the attack).
Hold Breath. The mosasaur can hold its breath for an hour.
Rampage. When the mosasaur reduces a creature to 0 hit points with a melee attack on its turn, the mosasaurus can take a bonus action to move up to half its speed and make a bite attack.
Siege Monster. The mosasaur does double damage to objects and structures.
Multiattack. The mosasaur makes two attacks: one with its bite and one with its tail or two with its tail.
Bite. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 34 (5d12 + 7) piercing damage. If the target is a Medium or smaller creature, it is grappled (escape DC 19). Until this grapple ends, the target is restrained and the mosasaur can't bite another target.
Tail. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 33 (4d12 + 7) bludgeoning damage.
Swallow. The mosasaur makes a bite attack against a Large or smaller creature it has grappled and the attack hits, the target is swallowed and the grapple ends. While swallowed, the target is blinded and restrained, it has total cover against attacks and other effects outside the mosasaur, and it takes 28 (8d6) acid damage at the start of each of the mosasaur's turns. A mosasaur can have only one creature swallowed at a time.
Unbridled Fury. In response to being hit by a melee attack, the mosasaur can make one melee weapon attack with advantage against the attacker.
The oceans of the dinosaurs' time were full of many large creatures, but few were as fearsome as the mosasaur. This 50-foot long reptile has the blubber of a whale, four powerful flippers, long, strong jaws, a second pair of teeth within the throat, and a long, powerful tail topped off with a dorsal fin of cartilage, much like the tail of a shark. It dwells in shallow reefs and lagoons as the apex predator, feeding on anything from fish, ammonites, sea turtles, giant sharks, and even smaller mosasaurs from time to time.