Undead Fortitude. If damage reduces the flesh builder drops to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is lighnting damage  or from a critical hit. On a success, the flesh builder drops to 1 hit point instead.

flesh building

as an action the flesh builder can turn a corpse within 5ft of them into a flesh builder.The original flesh builder and the new one gain 1d6 temp hp.

psychic disturbance 

creatures within 5ft of the flesh builder have disadvantage on wisdom saves.when the flesh builder  dies all creatures within 5ft of them take 2d6 psychic damage.

Slam. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) bludgeoning damage.