Crystal Guardian

Crackable Body.  If the golem takes bludgeoning damage equaling to 25 or more in a turn, its form is cracked. It loses the Immutable form ability and will spend its next turn fixing the crack, healing 2d12.

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

Light Refraction. Whenever the golem is subjected to lightning or radiant damage, it takes half damage and instead deals the same damage type equal to half the damage dealt to any creature within 10ft cube around the golem

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

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

Multiattack. The golem makes two slam attacks.

Slam. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage.

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.

Rush.- The golem moves up to its movement speed without provoking opportunity attacks

Slam. The golem makes a slam attack

Crystal Spikes (Costs 2 Actions). The golem slams its hands into the ground and in a 10 ft cube around the golem small spikes of gems start to show. During the next turn, any attack rolls against the golem are made with advantage, at the end of that turn any creatures within 10 feet of the golem take 5d6 piercing damage as the small crystals shoot from the ground and quickly retract back into the ground and the golem removes its hand from the ground. This second phase can be canceled by cracking the golem's body (see Crackable Body), at which case the golem removes its hands early to tend to the wound on its turn.