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

Magic Resistance. The tarrasque has Advantage on saving throws against spells and other magical effects.

Non-Physical: The Necrotic can move into and occupy the same space of other creatures. Other creatures can move into and occupy the same space as The Necrotic, and are not made prone when they do so.

Draining Presence:  Any creature that isn't an undead or construct that starts its turn within 60 feet of The Necrotic must succeed on a constitution saving throw or take 13 (2D12) necrotic damage and lose maximum hp equal to the damage taken (minimum of 0). The Necrotic heals by an amount equal to the damage dealt. Additionally, the ground within 15 feet of The Necrotic is considered difficult terrain for creatures that are not undead or constructs.

Necrotic Being: Whenever a creature (other than The Necrotic) rolls to do necrotic damage to The Necrotic, The Necrotic heals by an amount equal to the damage that would've been dealt if The Necrotic didn't have immunity to necrotic damage.

Reactive: The Necrotic regains use of its reaction whenever a creature starts its turn, rather than only when it starts its turn.

Shadow: If The Necrotic occupies a space of bright light or dim light, unless that light was created by a spell of 6th level or higher, or by an artifact, the light is suppressed and the space is covered by Darkness instead. If the light was created by one of those two things, the space is instead covered with dim light.

Multiattack. The Necrotic makes three Necrotic Blast attacks and uses Engulf. The Necrotic can replace any of these with the dash action.

Engulf. Every creature that is not an undead or construct within 0 feet of The Necrotic must make a DC 26 constitution saving throw. On a failed save, the creature takes 33 (5D12) necrotic damage, and their health decreases by an amount equal to the damage taken. On a successful save, the creature takes half as much damage only. The Necrotic heals by an amount equal to the damage dealt (minimum of 0).

Necrotic Blast. Ranged Attack Roll: +18, reach 120 ft. Hit: 24 (6D4 + 9) necrotic damage.

Dissipate. Trigger: The Necrotic is hit by an attack roll. Response: The Necrotic is no longer considered to occupy the space for the remainder of the turn. Additionally, The Necrotic can make a dexterity saving throw with a DC equal to the number rolled for the attack roll of the triggering attack. On a success, the attack misses.

Legendary Action Uses: 3. Immediately after another creature’s turn, The Necrotic can expend a use to take one of the following actions. The Necrotic regains all expended uses at the start of each of its turns.

Quick Blast: The Necrotic uses Necrotic Blast.

Reform (costs 2 actions.) Choose up to three adjacent spaces with at least one being adjacent to The Necrotic. Until the end of the next turn, The Necrotic is considered to occupy those spaces. The Necrotic uses Engulf.