Paragon Fortitude. The Decaying Flesh Golem has two pools of hit points, each of which is tracked separately. All damage and healing must be completely applied only to one pool of hit points. One pool of hit points must be completely reduced to zero hit points before any damage is applied to another pool. When a pool of hit points is reduced to zero, all ongoing conditions and effects affecting the creature end immediately. After a pool of hit points has been reduced to zero, it cannot receive any healing until after a long rest. If all pools of hit points have been reduced to zero hit points, the creature is killed.

Paragon Exhaustion. The Decaying Flesh Golem gains one complete turn in each combat round, including one reaction between each turn, for each pool of hit points it has above zero. When a pool of hit points has been reduced to zero, the creature loses one turn each round thereafter.

Berserk. Whenever the golem starts its turn with only one pool of hit points remaining, the golem goes berserk. On each of its turns while berserk, the golem attacks a random creature or object it can see which is near enough to move to and attack. Once the golem goes berserk, it continues to do so until it is destroyed or regains all its hit points.

The golem's creator, if within 60 feet of the berserk golem, can try to calm it by speaking firmly and persuasively. The golem must be able to hear its creator, who must take an action to make a DC 15 Charisma (Persuasion) check. If the check succeeds, the golem ceases being berserk. If it takes damage while still at 40 hit points or fewer, the golem might go berserk again.

Poisonous Bloat. The Decaying Flesh Golem is filled with a toxic gas. While the Decaying Flesh Golem has more than one hit pool remaining, whenever it takes slashing damage, some of this gas escapes. Any creature within 5 ft. must succeed on a DC 12 Constitution saving throw or take 1d6 poison damage.

On initiative count 20, if the Decaying Flesh Golem has only one pool of hit points remaining, any creature within 10 ft. must succeed on a DC 12 Constitution saving throw or take 1d6 poison damage. 

Aversion of Fire. If the golem takes fire damage, it has disadvantage on attack rolls and ability checks until the end of its next turn.

Immutable Form. The golem is immune to any spell or effect that would alter its form.

Lightning Absorption. Whenever the golem is subjected to lightning damage, it takes no damage and instead regains a number of hit points equal to the lightning damage dealt.

Multiattack. The golem makes two slam attacks.

Slam. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 13 (1d8 + 4) bludgeoning damage.