Corrupted Golem

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

Magic Resistance. The golem has advantage on saving throws against spells and other magical effects.

Magic Weapons. The golem's weapon attacks are magical.

Corruption.  The golem is corrupted by an aberrant plant-like creature.  This infection has grown within the golem and drives its actions.  Once the golem is brought below half-hp, the corruption bursts forth.  The golem loses access to slow and resistance to nonmagical damage.  It then gains vulnerability to fire and cold damage and access to its other features marked by *.

Multiattack. The golem makes two slam attacks.

Multiattack.* The golem makes two slam attacks and one bite attack.

Slam. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage.

Bite.* Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 17 (2d10 + 6)

Tongue Lash.* Melee Weapon Attack: +10 to hit, reach 15 ft., one target.  Hit: 13 (2d6 + 6)

Slow (Recharge 5–6). The golem targets one or more creatures it can see within 10 feet of it. Each target must make a DC 17 Wisdom saving throw against this magic. On a failed save, a target can't use reactions, its speed is halved, and it can't make more than one attack on its turn. In addition, the target can take either an action or a bonus action on its turn, not both. These effects last for 1 minute. A target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Debilitating Miasma* (Recharge 5-6). The golem emits noxious green gas in a 30 - foot cone.  Each creature must make a DC 17 Constitution saving throw, on a save the creature is unaffected, on a fail they are stunned until the end of the golem's next turn.

Once brought below half-hp, the golem can take 2 legendary actions, choosing from the options below.  Only one legendary action option can be used at a time and only at the end of another creature's turn.  The golem regains spent legendary actions at the start of its turn.

Detect. The golem makes a Wisdom (Perception) check.

Tongue Lash.  The golem makes a tongue-lash attack.