Colossus of the Elements (Core)

Unstable Energy. When the Elemental Colossus dies each creature within 50 feet of it must make a DC 18 Dexterity saving throw, taking 40 (12d6) true damage.

Elemental Immunity. The colossus core gets resistance to one type of damage it isn't already immune to. The damage type it is immune to changes each turn. 

Siege Monster. The colossus deals double damage to objects and structures.

Pure Energy. Ranged Weapon Attack: +15 to hit, range 60/240 ft., one target. Hit: 19 (2d8 + 10) true damage.

Elemental Chaos. The colossus core makes a elemental chaos attack at random.

1. Fire Breath. The colossus's head exhales a blast of fire in a 60ft cone. Each creature within the area must make a DC 18 Dexterity saving throw, taking 54 (12d8) fire damage on a failed save or half as much damage on a successful one. 

2. Tidal Slash. The colossus swings it's right arm slashing at all creatures in front of it. Each creature within 60ft must make a DC 18 Strength saving throw, taking 48 (8d12) cold damage and be pushed 60ft away from the colossus on a failed save, or half as much damage and only pushed 10ft on a successful one.

3. Poison Slam. The colossus slams it's left arm doing a slam attack. Any creature within 30ft of the targeted creature must make a DC 18 Constitution saving throw, taking 40 (8d10) poison damage on a failed save and half as much on a success. Creatures who fail the saving throw are also poisoned, creatures poisoned this way take 20 (4d10) poison damage at the end of their turn, they may repeat the saving throw afterwards clearing the effect on a success.

 4. Earthquake. A seismic disturbance radiates from the colossus in a 100ft radius. Each creature on the ground in that area must succeed on a DC 18 Dexterity saving throw or be knocked prone. Each creature on the ground that is concentrating must also succeed on a DC 18 Constitution saving throw or break concentration. The area becomes difficult terrain after the earthquake.

5. Whirlwind. The colossus summons a 10ft radius, 60ft tall whirlwind centered on a point within 120 feet of it. Each creature in that area must make a DC 18 Strength saving throw. On a failure, a target takes 27 (6d8) bludgeoning damage and in flung up to 40 feet away in a random direction and knocked prone.

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

Pure Energy. The colossus makes a pure energy attack

Legendary Action Name (Costs 2 Actions). The colossus regains use of elemental chaos.

Elemental Fusion (Cost 3 Actions). The core absorbs the energy of all nearby elementals dealing 20 (5d8) pure damage to elementals within 120ft of the core and healing the core equal to the damage dealt this way.