Cage Golem

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

Magic Resistance: The Cage Golem has advantage on saving throws against spells and other magical effects.

Spellcasting. The Cage Golem can cast the following spells innately (spell save DC 16, +7 to hit with spell attacks).
Cantrips (at will): Ray of Frost
1st level (3 slots): Earth Tremor, Grease
2nd level (2 slots): Web, Hold Person

Multiattack: The Cage Golem makes two fist attacks.

Fist Attack: Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) bludgeoning damage.

Sleeping Gas Breath (Recharge 5–6): The Cage Golem exhales sleeping gas, as per the Sleep spell cast at 9th level. ((22d8) hit points of creatures affected). The effect ends for a creature if it takes damage or someone uses an action to shake or slap the sleeper awake.

Cage Capture: The Cage Golem attempts to capture a medium or smaller creature within 5 feet of it in its cage torso. The target must succeed on a DC 15 Dexterity saving throw or be captured and restrained inside the cage. A creature, including the one restrained, can use its action to make a DC 18 Strength check, freeing only itself or another creature within the cage on a success. The cage can hold up to four Medium or smaller creatures.

Retaliatory Capture: When a creature within 5 feet of the Cage Golem hits it with a melee attack, the golem can try to capture that creature as a reaction, forcing the creature to make a DC 15 Dexterity saving throw or be captured and restrained as described in Cage Capture.