Smaragdine Golem

Construct Nature. The golem doesn't require air, food, drink, or sleep.

Enchanted Weapons. The golem's weapon attacks are magical. When the golem hits with any weapon, the weapon deals an extra 5d8 force damage (included in the attack).

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 Vessel. When the smaragdine golem counters or ends a spell with Absorb Spell, it holds the spell's magical energy within its body and becomes Charged for 1 minute. While Charged, the golem sheds dim light in a 10-foot radius.

Multiattack. The smaragdine golem makes two Slam attacks. It can replace one attack with a use of Release Spell.

Slam. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 20 (3d8 + 7) bludgeoning damage plus 22 (5d8) force damage.

Release Spell (Recharge Special). The smaragdine golem releases an absorbed spell's energy in a green burst centered on a point the golem can see within 60 feet of it. Each creature, other than the smaragdine golem, within 5 feet of that point per level of the absorbed spell must make a DC 18 Dexterity saving throw. On a failure, a creature takes 10 (3d6) force damage per level of the absorbed spell and is incapacitated until the end of its next turn. On a success, a creature takes half the damage and isn't incapacitated. The golem can use this action only if it is Charged (see the Magic Vessel trait).

Absorb Spell. When a creature the smaragdine golem can see within 30 feet of it casts a spell, the golem can absorb the spell's energy, countering it. This works like the counterspell spell, except the golem must always make a spellcasting ability check, no matter the spell's level. Alternatively, when a creature the golem can see within 30 feet of the golem starts its turn while under the effects of a spell, such as bless or heroes' feast, the golem can absorb the spell's magic, ending the effect. In either case, the golem's ability check for this is +10. If it successfully counters or ends the spell, it becomes Charged (see the Magic Vessel trait). The golem can't use Absorb Spell while it is Charged.