Cildraess, The Caller of Ruins

Deathly Ruin. When Cildraess is reduced to 0 hit points, its internal elemental energies detonate. All creatures within 120 ft. of Cildraess when it is reduced to 0 hit points must succeed on a DC 24 Dexterity saving throw or take 110 (20d10) force damage, or half as much damage on a success.

Legendary Resistance (3/Day). If Cildraess fails a saving throw, it can choose to succeed instead.

Multi-Headed. Cildraess has three heads, with each one corresponding to a specific type of damage - blue for cold damage, red for fire damage, and yellow for lightning damage. While it has more than one head, it has advantage on Wisdom (Perception) checks it makes, as well as advantage on saving throws against being Charmed, Deafened, or Frightened. 

If Cildraess takes 50 damage or more in a single turn, one of its heads dies. While a specific head is dead, Cildraess' attacks cannot use the damage type associated with that head. Cildraess can survive without any of its heads being alive.

Reactive Heads. For each additional head Cildraess has, it can take one additional reaction, which can only be used to make opportunity attacks.

Hydric Regeneration. At the start of Cildraess' turns, it regains 30 hit points. If Cildraess has any dead heads (see "Multi-Headed" above), it regrows one head at the end of its next turn. This trait doesn't function when Cildraess drops to 0 hit points.

Multiattack. Cildraess uses its Frightful Presence, then makes up to five melee attacks: two with its Claws, and one Bite attack for each head it has alive. If available, it can also use its Elemental Breath. It can replace its Claws attacks with its Bi-Elemental Spear.

Bite. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 23 (3d10+7) piercing damage plus 13 (2d12) fire, cold, or lightning damage, depending on which head is used for the attack.

Claws. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 20 (3d8+7) slashing damage.

Bi-Elemental Spear. Ranged Spell Attack: +12 to hit, range 120 ft., two targets. Hit: 19 (3d12+6) fire, cold, or lightning damage.

Elemental Breath (Recharge 5-6). Cildraess exhales its elementally charged breath in a 120 ft. cone. Each creature within that cone's area must make a DC 22 Dexterity saving throw, taking 97 (15d12) fire, cold, or lightning damage, based on which head Cildraess uses for the attack, or half as much damage on a success.

Frightful Presence. Cildraess exudes an aura of terror. Each creature within 120 ft. of Cildraess that is aware of it must succeed on a DC 22 Wisdom saving throw or be Frightened for 1 minute. A frightened creature can repeat its saving throw at the end of each of its turns, ending the effect on itself on a success. A creature that succeeds on its saving throw, or has the effect end for it, is immune to Cildraess' Frightful Presence for the next 24 hours.

Cildraess 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. Cildraess regains spent legendary actions at the start of its turn.

Head Lash. Cildraess makes a Bite attack.

Elemental Pillar (Costs 2 Actions). Cildraess chooses up to three 10 ft. cubes within 60 ft. of itself. Any creatures within the area of the cubes must make a DC 16 Dexterity saving throw or take 13 (2d12) fire, cold, or lightning damage.