Beles, The Unbound

Amphibious. Beles can breathe air and water.

Infused Scales. Any time Beles is targeted by a Magic Missile spell, a line spell, or a spell that requires a ranged Attack roll, roll a d6. On a 1 to 6, the Beles is unaffected and instead unleashes a special ability according to the roll.

1. Eruption: The magic charges the scales which detonate, each creature in a 10-foot radius centered on Beles must succeed on a DC 23 Dexterity saving throw or take 18 (4d8) force damage.
2. Rejuvenation: Beles regains a number of hit points equal to 4 times the level of the spell targeting her.
3. Chaotic Growth: The terrain in a 30-foot radius centered on Beles becomes overrun with crystalline plants and vines until the end of her next turn. A creature other than Beles moving through the area must spend 4 feet of movement for every 1 foot it moves.
4. Crystallize: Beles' scales toughen. She gains resistance to
   all damage until the start of her next turn.
5. Recharge Power: The magic recharges Beles' Unleash Magic.
6. Reflect: Beles is unaffected, and the effect is reflected back at the caster as though it originated from Beles, turning
   the caster into the target.

Legendary Resistance (3/day).  If Beles fails a saving throw, she can choose to succeed instead.

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

Magic Infused. Beles' attacks are magical.

Slithering Form. Beles has resistance to damage from attacks of opportunity.

Multiattack. Beles makes two attacks: one with its bite and one with its constrict.

Bite. Melee Weapon Attack; .17 to hit. reach 10 ft.. one target. Hit: 22 (3d8 + 9) piercing damage. If the target is a Large or smaller creature, it must succeed on a DC 25 Dexterity saving throw or be swallowed by Beles. A swallowed creature is blinded and restrained, it has total cover against attacks and  other effects outside Beles, and it takes 21 (6d6) force damage  at the start of each of the Beles' turns.

If the Beles takes 45 damage or more on a single turn from a
creature inside it, Beles must succeed on a DC 22 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of Beles. If Beles dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 20  feet of movement, exiting prone.

Constrict. Melee Weapon Attack: +17 to hit, reach 10 ft., one
creature. Hit: 25 (3d10 + 9) bludgeoning damage, and the target is grappled (escape DC 25). Until this grapple ends, the creature is restrained. Beles can constrict up to 4 Large creatures, or 8 Medium ones. Beles can make this attack again on creatures it is already restraining, automatically hitting if it chooses to do so.

Natural Cataclysm. Beles unleashes the energy held within her body, summoning a natural disaster. Roll 1d4, According to the result a cataclysm is chosen, dictating the damage type:

1. Cyclone (slashing),
2. Volcanic eruption (fire)
3. Tsunami
   (bludgeoning),
4. Blizzard (cold),

Each creature other than Beles in a 150 foot radius centered on Beles must succeed on a
DC 25 Constitution saving throw or take 22 (5d8) of the resulting damage.

Unleash Magic (Recharge 5-6). Beles exhales raw magic in a 120--foot line that is 10 feet wide. Each creature in that line must make a DC 23 Dexterity saving throw, taking 88 (16d10) force damage on a failed save, or half as much damage on a successful one.

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

Coil. Beles uses its constrict attack.

Move. Beles shakes its body. Each creature within 15 feet of Beles must succeed on a DC 25 Strength saving throw or take 18 (2d8 + 9) bludgeoning damage and be knocked prone. Beles can then move up to half its speed.

Beles' Wrath. (Costs 2 Actions). A storm made up of destructive nodes of condensed magic appears in a location Beles chooses within 150 feet. The area of the storm consists
of up to eight 10-foot cubes, which you can arrange as you wish. Each cube must have at least one face adjacent to the face of another cube. Each creature in the area must make
Dexterity saving throw, It takes 6d10 force damage on a failed save, or half as much damage on a successful one.