Undead Fortitude. If damage reduces the monster 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 monster drops to 1 hit point instead.

Size Advantage. +1 on attack rolls for each size larger then the target, but decrease one size permanently when taking critical hits. Can regain size and hit points by eating corpses (Heal equal to the maximum number of hit points the eaten corpses posses, if the monster heals more than 100 hit points it can regain one size.

Poison Flesh. whenever a creature stop within 15 feet of the monster they make a DC 16 Constitution saving throw, taking (4d8) poison damage on a failed save, or half as much damage on a successful one.

Slam. Melee Weapon Attack: +15 to hit, reach 5 ft., one target. Hit: (3d12+10) bludgeoning damage, or (1d12+10) piercing damage if the swarm has half of its hit points or less.

Plague Vomit (Recharge 6). The Monster exhales nauseous fumes in a 90-feet cone. Each creature in that area must make a DC 23 Constitution saving throw. A creature takes (6d10) poison damage on a failed save, or half as much damage on a successful one. A creature that fails the save by 5 or more is poisoned.