Arena Golem

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 Weapons. The golem's weapon attacks are magical. 

Carnage: The golem uses one of the following attacks(roll 1d8 or choose one). The golem can’t use the same attack twice in a row. 

1. Pummel: Melee weapon attack, +7 to hit, reach 5ft, one target. Hit 17(3d8+4) bludgeoning damage and the target must succeed on a DC 15 Strength saving throw or be knocked Prone.
2. Cleave: Melee weapon attack, +7 to hit, reach 5ft, one or two targets that are within 5ft of each other and the golem. Hit 23(3d12+4) slashing damage. 
3. Multiattack: The golem makes three Hack attacks.
4. Stomp: The golem makes this attack with disadvantage against targets that aren’t knocked Prone. Melee weapon attack, +7 to hit, reach 5ft, one target. Hit 32(8d6+4) bludgeoning damage.
5. Chain Release: The golem launches a spiked chain that is 30ft long and 5ft wide, Each creature in that area must make a DC 15 Dexterity saving throw. On a failed save, a creature takes 14(3d6+4) piercing damage. 
6. Bellow: The golem lets out a loud metallic roar. Each creature of the golem’s choice that is within 30ft of it and can hear it must succeed on a DC 12 Wisdom saving throw or become Frightened until the end of the golem’s next turn. 
7. Fire Breath: The golem exhales fire in a 30ft cone. Each creature in that area must make a DC 12 Dexterity saving throw, taking 17(5d6) fire damage on a failed save, and half as much damage on a successful one. 
8. Encage: Melee weapon attack, +7 to hit, reach 5ft, one medium or smaller creature. Hit: the target is grappled(escape DC 15). Until this grapple ends, the target is Restrained, and the golem can’t grapple another target or attack the grappled creature. In addition, at the start of each of the target’s turns, the fires churning inside the golem burn the target, dealing 10(3d6) fire damage.

Hack: Melee weapon attack, +7 to hit, reach 5ft, one target. Hit 7(1d6+4) bludgeoning damage. 

The Arena Golem can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. The Arena Golem regains spent legendary actions at the start of its turn. 

Hack: The Arena Golem makes a Hack attack. 

Charge: The Arena Golem moves up to its speed straight toward a target, moving through every medium or smaller creature in its way. Every creature it moves through takes 10(3d6) bludgeoning damage and must succeed on a DC 15 Strength saving throw or be knocked Prone. 

Spin Chain: Each creature within 5ft of the Arena Golem must succeed on a DC 15 Strength saving throw or be knocked Prone.