Calasmos

Legendary Resistance (6/Day). If Calasmos fails a saving throw, it can choose to succeed instead.

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

Necrotic feed. The Calasmos feeds off any and all necrotic energy. If he takes any necrotic damage, he will heal for the exact amount he would have taken

Cloaked in Darkness. so long as he has this active he has this active he is Resistant to all magic that doesn't deal radiant damage and takes half damage from melee attacks(to get rid of this need to use a lvl 9 Dispell magic for 1d4 turns,  Sunburst at lvl 9, or Crown of Stars to cleanse the darkness for 1d8 turns)

Immensity. Calasmos can move through the space of a creature or object of Gargantuan size or smaller, and it does not treat such creatures as difficult terrain. If the titan moves through a creature's space, the creature must make a DC 20 Dexterity saving throw, taking 55 (10d10) bludgeoning damage on a failure. A creature can only be made to make this save once per round

Come Unto Your Maker. When Calasmos is reduced to 2/3 of its hp maximum and then again reduced to 1/3 of its hp maximum once after a long rest, Calasmos Immediately kills 1 character. This feature does not trigger if the party has less then 4 members for the first proc, or less then 3 members for the second proc.

Hates the Light. Takes double damage from radiant damage sources. on a critical he takes triple damage

Regeneration. Calasmos does regenerate +30 hp every round

Magic Weapons. Calasmos weapon attacks are considered magical.

Innate Spellcasting. Calasmos innate spellcasting ability is Intellect (spell save DC 28, +20 to hit with spell attacks). It can innately cast the following spells, requiring no materials:

At will: create or destroy water, detect evil and good, detect magic, fog cloud, greater restoration, stone shape, thunderwave
5/day: commune, counterspell, dispel evil and good, dispel magic, identify, move earth, passwall, water walk, wind walk
3/day: creation, invisibility, plane shift, wall of fire, wall of stone
1/day control weather, earthquake

Multiattack. Calasmos makes three attacks with its claws, one with its cannon and one slam attack.

Claw. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 42 (10d12 + 16) slashing damage.

Cannon. Ranged Weapon Attack: +20 to hit, reach 200ft, radius 90ft. Hit 50 (10d12+16) Force Damage

Slam. Melee Weapon Attack: +21 to hit, reach 30 ft., one target. Hit: 120 (20d10 + 10) bludgeoning damage.

As a bonus action after Calasmos can fire a Darkling from its cannon but only after losing half its health. When the Darkling enters battle roll 1d4 it is unresponsive for that many turns. on the next turn after turns rolled it will cast MASS HEAL targeting itself and Calasmos. after casting the spell it will go dormant for 1d4 turns and repeat.

• as a bonus action Calasmos can make a breath attack from an opening on its body. Breath Attack: Calasomos exhales in a 120-foot cone. Each of the creatures in that area must make a DC 30 Dexterity saving throw or take 80 (8d20) damage on a failed save and half as much on a successful one.
• to determine the type of damage dealt you will roll a 1d8. (1. Poison, 2.Fire 3.Frost 4.Force 5.Lightning 6.Thunder 7.Necrotic 8.Psychic)

Magic Reflection: When Calasmos is attacked with a magic spell, half of the damage dealt is reflected back at the caster. (radiant damage is not reflected)

Mythic reactions. Calasmos can make six reactions each round, four with its claw attacks, one with

Calasmos can take 10 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. Calasmos regains legendary actions on every other turn of his after the first one is used.

(1 Charge) -Claw Attack

(1 Charge) -Slam Attack

(2 Charges) -Breath Attack

(5 Charges) - Cast Earthquake at 9th lvl

1 Time only, when reaches 250HP remaining will use 12th level Spell Unique to him

The Grip of Death: The DM will Roll 1D8 and than randomly pick 8 targets in the combat that are his enemies. the targets of the spell die instantly and cannot be revived.