Clockwork Machima

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

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

Brachium. When the Clockwork Machima hits 0 hit points, it will launch a wave of magical energy out before shutting down. Every creature, excluding its master, in a 15ft circle around the Machima must make a DC 15 CON or DEX save. A creature takes 3d10 force damage on a failed save, or half as much on a passed save.

Magical Machinery. Once the Machima has powered-down, a creature can use 1 spell slot of any level to reanimate the Machima. The Machima is them powered on with an amount of hit points equal to twice the level of the spell slot used. The Machima can enter a rest mode, and recover 1d6 hit points per hour spent in this mode.

The Machima cannot attack, but can use its action to either interact with an object or protect its master. 

If the Machima is within 5 ft of its master, it can use its reaction to defend them, making a DC 10 Dexterity check.