Cold Iron Golem

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

Aura of Malaise: The Golem emanates an aura in a 20 foot sphere around it; the air appears to wave and shimmer slightly within the sphere - creatures outside the sphere targeting creatures within the sphere deduct 1d6 from their attack roles. Additionally, creatures within the aura who attack more than once on their turn are stunned until the end of their current turn - this occurs when the second attack is rolled, but before it is determined if it hits - thus automatically making that second attack miss. Constructs are immune from this effect. 

Dealing lightning damage suppresses the aura until the start of its next turn. 

Desperate Surge: Once per day, when the golem ends its turn below 20% of its maximum hit points, it can take another full turn. 

Misty Step: The Cold Iron Golem can cast Misty Step 3 times per short rest. 

Magical Regeneration: When the Golem starts its turn on more than 0 hit points, it regenerates 1d4 hit points. (DM Note, to save you from rolling while out of combat, you can assume Golem regenerates 25 hit points per minute using this feature). 

Multiattack: The Cold Iron Golem can make one melee attack and one ranged attack on its turn.

Melee Attack: +10 to hit, reach 5 ft., one target. Hit: 24 (6d8 + 5) bludgeoning damage. Creatures struck by this must succeed a DC 16 Strength check or be knocked prone. 

Ranged Attack: +10 to hit (60/300 range)., one target. Hit: 20 (3d10 + 5) piercing damage.

The Cold Iron Golem can take 1 legendary action, which can be used to make one attack (melee or ranged, its choice).