homebrew
Undead Fortitude. If damage reduces the seafallen to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the seafallen drops to 1 hit point instead. WeakIy Armored. Whenever the seafallen takes more than 10 damage in a single turn, a piece of its coral armor chips off,reducing its AC by 1. This reduction is cumulative, but cannot reduce the seafallen's AC below 10.
Slam. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6+1) 1d6+1 bludgeoning damage. Water Whip (1/day). The seafallen releases a line of water in a 10-foot line that is 5 feet wide. Each creature in that line must make a DC 11 Dexterity saving throw. On a failure, a target takes 7 (2d6) 2d6 bludgeoning damage and, if it is Medium or smaller, is pushed up to 5 feet away from the seafallen. On a success, a target takes half as much damage and isn't pushed.
Environments. Coastal, Ocean, Sea, Underwater