Loathsome Limbs (4/Day). If the world eater ends any turn bloodied and took 15+ Slashing damage during that turn, one of the world eater’s limbs is severed, falls into the world eater’s space, and becomes a troll limb. The limb acts immediately after the world eater’s turn, it grows replacement limbs the next time it regains Hit Points.

Regeneration. The world eater regains 15 Hit Points at the start of each of its turns. If the world eater takes acid or force damage. If the world eater takes acid damage, it regains only 5 hit points at the start of its next turn. The world eater only dies only if it takes 20 or more acid damage while it has 0 hit points.

Rancid Aura. While it's alive, the world eater generates an aura of decay that fills the area within 30 feet of it. At the start of the troll's turn, all nonmagical flames in the aura are extinguished, Sources of light in a 120-foot emanation originating from the world eater flicker wildly. Nonmagical sources of Bright Light in that area instead shed Dim Light. Any creature of the world eater's choice that starts its turn within 30 feet of the troll takes 10 (3d6) cold damage and 11 (2d10) necrotic damage must succeed on a DC 17 Wisdom saving throw or become frightened of trolls and world eaters. A frightened target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a target's saving throw is successful or the effect ends for it, the target is immune to the frightened effects of the aura for the next 24 hours. The ground is also Difficult Terrain The world eater can magically command any troll within 120 feet of it, using telepathy and trolls are also unaffected by the aura.

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

Hexa-Headed. The world eater has advantage on saving throws against being stunned and knocked unconscious and magic can't put the world eater to sleep. The world eater also can't be surprised, 

Corrosive Form. Nonmagical ammunition is destroyed immediately after hitting the world eater and dealing any damage. Any nonmagical weapon takes a cumulative -1 penalty to attack rolls immediately after dealing damage to the world eater and coming into contact with it. The weapon is destroyed if the penalty reaches -5. The penalty can be removed by casting the Mending spell on the weapon. The world eater's skin can eat through 2-inch-thick, nonmagical metal or wood in 1 round. Any creature grappling the world eater or grappled by it takes 6 (1d12) acid damage at the end of that creature's turn. The world eater can move through a space as narrow as 1 inch without squeezing and when the troll takes damage of any type but psychic, each creature within 5 feet of the troll takes 9 (2d8) poison damage.

Damage transfer. while grappling a creature they world eater can transfer half the damage it takes to a creature it has grappled

Fling Limb (3/Day). Ranged Weapon Attack: +13 to hit, range 60/240 ft., one target. Hit: 11 (1d8 + 7) bludgeoning damage. If the limb hits a creature, that creature has the grappled condition (escape DC 17). The limb has the statistics of a world eater, except for the following: it is Medium, it has 45 hit points, its speed is 30 ft., it doesn't have a challenge rating or Legendary Resistance, and the only action it can take is the Attack action, which it can use only to grapple. Until this grapple ends, the target has the restrained condition and takes 25 (4d8 + 7) slashing damage at the start of each of its turns.

Rampage (2/Day). Immediately after dealing damage to a creature that is already Bloodied, the world eater moves up to half its Speed, and it makes one claw attack. 

Bite. As part of its Multi-attack, the world eater can make a Bite attack. Melee Weapon Attack: +17 to hit, reach 5 ft., one target. Hit: 29 (6d6 + 8) piercing damage plus 21 (3d10 + 5) psychic damage, and the target must succeed on a DC 24 Wisdom saving throw or be stunned for 1 minute. The stunned target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If the target is a creature that isn't a Construct or an Undead, the world eater regains Hit Points equal to the damage dealt. The target dies if it is reduced to 0 Hit Points by this attack. Its body is then absorbed into the world eater, leaving only equipment behind, and the world eater gains any traits the creature may have had that the world eater does not or improves upon a trait it has.

Children of the troll (1/Day). The world eater magically summons 1d4 + 2 dire trolls in unoccupied spaces it can see within 30 feet of it. The trolls are under the world eater's control and act immediately after the world eater in the initiative order. The trolls disappear after 1 hour, when the world eater dies, or when the world eater dismisses them (no action required). 

Unbridled Fury. In response to being hit by a melee attack, the world eater can make one Bite or claw attack with advantage against the attacker. The world eater can teleport to an unoccupied space it can see within 30 feet of it after the attack hits.

Spell Reflecting Ray. When the world eater takes damage from a spell, the world eater takes only half the triggering damage. If the spellcaster is within 60 feet of the world eater, the world eater can force the caster to make a DC 22 Dexterity saving throw. Unless the save succeeds, the caster takes the other half of the damage and is pulled up to 30 feet straight toward the world eater. if the world eater succeeds on a saving throw against a spell, or a spell's attack roll misses it. if the world eater succeeds target one creature within 120 ft. Dexterity Saving Throw: DC 22 Failure: 10 (3d6) Force damage.

3 legendary actions

Tentacles. (1 action) Melee Weapon Attack: +17 to hit, reach 15 ft., one creature. Hit: The target is grappled (escape DC 25), and the world eater pulls the target up to 15 feet straight toward it. Until this grapple ends, the target takes 5 (1d10) lightning damage at the start of each of its turns, and the world eater can't use its tentacles on another creature. It has two tentacles

Web Bomb. (2 actions) Ranged Weapon Attack: + 15 to hit, range 300 ft./600 ft., one target. Hit: 24 (3d10 + 8) bludgeoning damage, and the target and all creatures within 10 feet of it must succeed on a DC 19 Dexterity saving throw or take 10 (3d6) acid damage and have the restrained condition until the start of the spider's next turn. 

Shape Theft. (1 action) After the world eater kills a Humanoid, the world eater can magically transform itself to look and feel like that creature while retaining its game statistics (other than its size). This transformation lasts until the world eater dies or uses an action to end it.

Stinger. Melee Weapon Attack: +10 to hit, reach 10 ft., one creature. Hit: 22 (3d10 + 6) piercing damage, and the target must succeed on a DC 25 Constitution saving throw or have the poisoned and paralyzed conditions for 1 minute. The affected creature can repeat the saving throw at the end of each of its turns, ending both the poisoned and paralyzed conditions on itself on a success.

Eat Memories. (2 actions) The world eater targets one creature it can see within 5 feet of it. The target must succeed on a DC 15 Wisdom saving throw or take 18 (4d8) psychic damage and become memory drained until it finishes a short or long rest or until it benefits from the greater restoration or heal spell. Constructs, Oozes, Plants, and Undead succeed on the save automatically.

While memory drained, the target must roll a d4 and subtract the number rolled from its ability checks and attack rolls. Each time the target is memory drained beyond the first, the die size increases by one: the d4 becomes a d6, the d6 becomes a d8, and so on until the die becomes a d20, at which point the target becomes unconscious for 1 hour. The effect then ends.

The world eater learns all the languages a memory-drained target knows and gains all its skill proficiencies

Decaying Impersonation. (3 actions) The world eater extrudes a piece of itself that assumes the appearance of one creature whose memories it has stolen. This simulacrum appears, feels, and sounds exactly like the creature it impersonates, though it smells faintly of decay. The world eater can impersonate 1d4 + 1 different creatures, each one tethered to its body by a psychic link that can extend up to 120 feet away. The simulacrum is an extension of the world eater. The simulacrum disappears if the psychic link is severed.