Burradur, The Flesh Collective

Undead Fortitude. If damage reduces Burradur to 0 hit points, it must make a constitution saving throw with a DC of 5 +the damage taken, unless the damage was radiant or from a critical hit. On a success, Burradur drops to 1 hit point instead.

...Their Armor Against Them. Any time Burradur is targeted by a miagic Missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. on a 1 to 5, Burradur is unaffected. On a 6 Burradur is unaffected and the effect is reflected back at the caster as though it originated from Burradur, turning the caster into the target.

Freedom of Movement. Burradur ignores difficult terrain, and magical effects can't reduce its speed or cause it to be restrained. 

Spider Climb. Burradur can climb difficult surfaces, including upside down on ceilings, without needing to make an ability Check.

Siege Monster. Burradur deals double damage to objects and structures.

True Form. Burradur is unaffected by effect that would alter its form. (Polymorph, True Polymorph, etc.)

Multiattack. Burradur makes four tentacle attacks, each of which it can replace with one use of Fling.

Bite. Melee Weapon Attack: +17 to hit, reach 5 ft., one target. Hit: 23 (5d8 + 10) piercing damage. If the target is a Large or smaller creature grappled by Burradur, that creature is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside Burradur, and it takes 42 (12d6) Necrotic damage at the start of each of Burradurs turns.

If Burradur takes 60 damage or more on a single turn from a creature inside it, Burradur must succeed on a DC 25 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of Burradur. If Burradur dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 15 feet of movement, exiting prone.

Tentacle. Melee Weapon Attack: +17 to hit, reach 30 ft., one target. Hit: 20 (5d8 + 10) bludgeoning damage, and the target is grappled (escape DC 18). Until this grapple ends, the target is restrained. Burradur has ten tentacles, each of which can grapple one target.

Fling. One Large or smaller object held or creature grappled by Burradur is thrown up to 60 feet in a random direction and knocked prone. If a thrown target strikes a solid surface, the target takes 3 (1d6) bludgeoning damage for every 10 feet it was thrown. If the target is thrown at another creature, that creature must succeed on a DC 18 Dexterity saving throw or take the same damage and be knocked prone.

Bone Spurs. Burradur creates three bolts of bone and metal, each of which can strike a target Burradur can see within 120 feet of it. A target must make a DC 23 Dexterity saving throw, taking (5d10) Piercing damage on a failed save, or half as much damage on a successful one. 

Tomb Breaker. Burradur slams the ground and creates a shockwave that blasts out 60' from the center. each creature must make a DC 23 Dexterity saving throw, taking (8d10) bludgeoning damage and knocked prone on a failed save, or half as much damage on a successful one. The ground in this area becomes difficult terrain.

For The Greater Good. As a reaction, Burradur can shove off chunks of itself to block an attack. When this happens, Burradur loses 2 Hit Dice worth of health. (2d20)

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

Tentacle Attack or Fling. Burradur makes one tentacle attack or uses its Fling.

Bone Spurs(Costs 2 Actions). Burradur uses Bones Spurs.

Refresh The Flesh. (Must have a corpse within 30 feet). Every time Burradur consumes a corpse, it adds a Hit Dice worth of health back into its Hit Point pool. (2d20)

Rancid Release (Costs 3 Actions). Burradur expels a greenish black cloud in a 60-foot radius. The cloud spreads around corners, and that area is heavily obscured to creatures other than Burradur. Each creature other than Burradur that ends its turn there must succeed on a DC 23 Constitution saving throw, taking 20 (8d10) necrotic damage on a failed save, or half as much damage on a successful one. A strong current disperses the cloud, which otherwise disappears at the end of Burradurs next turn.

Burradur has mythic actions for one hour after consuming enough corpses to gain 100 Hit Points.

Bring Out Your Dead (Costs 2 Actions). Burradur can raise all dead within a 20'x20' area within 120 of his vision. These undead have the stats of a Zombie and are permanently under Burradurs control. (These can be added to Burradur via Refresh The Flesh.) 

Cacophonous Sarcophagus (Costs 3 Actions). Burradur Spits a glob of itself at an enemy. The creature must succeed on a DC 23 Strength Saving Throw or be grappled in the mass. The Creature must then succeed on a DC23 Wisdom saving throw or take 5d10 Psychic damage, or half as much on a success, until it escapes. The creature can make another strength saving throw at the beginning of its turn to escape, but must make Wisdom saves at the end of its turn until it escapes.

Ungodly Thrash (Costs 3 Actions and loses Mythic ability until requirement is met again). Burradur makes an attack for each remaining tentacle. (Can direct up to 5 attacks per target.)