Batriel the Astral Deity

Special Trait Name.  Siege monster. Batriel does extra damage for structures. 

Legendary resistance. If he Batriel fails a saving throw he can choose to succeed. 

Unusual Nature. Batriel does not require air. 

Nebulas Thoughts. Any magic attempt to read Batriel's mind or glean automatically fail. 

Regeneration. Batriel regains 20 hit points at the start of his turn. 

Devils sight. Magical darkness has no effect on Batriel. 

Amphibius. This creature can breathe both air and water.

Earth Glide. Batriel can glide through nonmagical and unworked stone 

Keen Sight. Batriel has advantage on Wisdom (Perception) checks that rely on sight.

Spellcasting.  

Meteor swarm. 

Earthquake 

Sunburst. 

Fire Storm. 

Bless. 

Water Breathing. 

Phase 2 abilities and spells. 

Pack tactics.  Batriel has advantage on an attack role against a creature if one of his allies are within 5 feet and if the ally isn't incapacitated. 

Watery advantage. While submerged in liquid Batriel gains advantage on attack roles. 

Death Throes. When Batriel dies, it explodes, and each creature within 30 feet of it must make a DC 20 Dexterity saving throw, taking 70 (20d6) fire damage on a failed save, or half as much damage on a successful one. The explosion ignites flammable objects in that area that aren’t being worn or carried, and it destroys Batriel's body and possessions.

Multiattack. Batriel takes one bite and one horn attack. 

Chromatic Flames. Batriel exhales multi-colored flames in a 350-foot cone. Each creature in that area must make a DC 27 dexterity saving throw. On a failed save the creature takes 71(11d12) damage of a type that he chooses, acid, cold, fire, lightning, or acid. On a successful save the creature takes half as much damage. (Recharge 5-6)

Bite. melee weapon attack. + 19 to hit, reach 15 feet. One target. Hit 23 (2d12 +10) piercing damage plus (4d6 +10) force damage. 

Horns. Melee Weapon Attack: +19 to hit, reach 10ft., one target. Hit: 32 (4d10 + 10) piercing damage. 

 Flame explosion. Batriel lets a large flaming explosion come off of his scales and any creature within a 100-foot radius must make a DC 23 Dex saving throw or take (9d10) fire damage on a failed save. On a successful save the creature takes half damage. 

Tail. Melee Weapon Attack: +19 to hit, reach 20ft., one target. Hit: 24 (4d6 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.

Photonic breath. The creature exhales a flashing mote of radiant energy to a point that he can see within 240 feet and 40-foot radius. Each creature in the area must make a DC 23 constitution saving throw, taking 66(12d10) radiant damage or half on a successful save. Recharge 5-6. 

Phase 2 actions. 

Lightning Storm. Batriel magically creates three bolts of lightning, each of which can strike a target that Batriel can see within 120 feet of it. A target must make a DC 23 Dexterity saving throw, taking 22 (4d10) lightning damage on a failed save, or half as much damage on a successful one.

Deathly ray.  Ranged spell attack. +12 to hit, range 120 feet one target. Hit 30 (7d6 +6) Necrotic damage. 

Move. Batriel can move his movement speed in any direction any movement type as a bonus action. 

Reflective skin. If Batriel is attacked with magic missile, then the missiles all reflect back and hit the creature who cast it.  

Stomp.  Batriel stomps his massive foot crumbling the ground and shaking the plane of existence they are one. All creatures on the ground must make a DC 22 strength saving throw. 

Phase 2. Once Batriel is reduced to 400 hp, he gains new abilities and attacks. 

Phase 3. Batriel  teleports all creatures in combat with him to a Colosseum in the empty void and the only way to leave would be to defeat him or a spell that allows you to leave like "Wish" or "Plane Shift".