Clockwork Golem Sentinel

Fire Absorption. Whenever the golem is subjected to fire damage, it takes no damage and instead regains a number of hit points equal to the fire damage dealt.

Lightning Absorption. Whenever the golem is subjected to lightning damage, it takes no damage and regains a number of hit points equal to the lightning damage dealt. 

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

Clockwork Construction. Once per day the golem must be mechanically wound up in order to function. Once wound up the golem is immune to exhaustion.

Self Destruct. When the golem is reduced to 10% of it's starting hit points or less, the golem begins a self destruct sequence. The golem spends the round sounding an alarm to alert any other guards. On the next round the golem self destructs. Any creatures within a 20 foot radius must make a DC 16 Reflex saving throw, taking 6d6 fire damage on a failed save, or half as much on a successful one.

Multiattack. The golem makes two melee attacks.

Slam. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit:  (3d8 + 6) bludgeoning damage.

Clockwork Grind Saw. Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: (3d10 + 6) slashing damage.

Fire Breath (Recharge 6). The golem exhales fire in a 15-foot cone. Each creature in that area must make a DC 16 Reflex saving throw, taking  (4d8) fire damage on a failed save, or half as much damage on a successful one.

Lightning Fist (Recharge 5). The golem slams forward an arc of lightning from it's fist in a straight line forward for 15 feet. Each creature in range must make a DC 16 Reflex saving throw, taking 4d6 lightning damage on a failed save, or half as much damage on a successful one.