Tentacle Horror

Immutable Form : The horror is immune to any effects that would alter its form.

Legendary Resistance 2/day: If the horror fails a saving throw it can choose to succeed instead.

Tentacled Horror : The Horror Spawns with 8 tentacles, and is capped at 16 tentacles.
Only 4 tentacles are ever on the surface(Active) at one time, any exceeding this active cap are under ground, they are immune to damage and untargetable while under ground.

The main body is immune to damage while there are active tentacles on the surface, 

Each tentacle has 35hp 17AC and 15ft reach, , each takes their own turn but act on the Horror's initiative. when a tentacle is destroyed the main body is dealt half of the damage of the killing blow.

The tentacle can use its action to Attack (tentacle attack), Dodge or grapple.
If the tentacle manages to grapple a target, it may restrain the target and force it prone

While The target is prone the Tentacle may use its action to constrict the target, causing 2d6 bludgeoning damage

the target may attempt to break the grapple at the end of its turn.

the tentacles use the monsters saving throws.

Maw - The Creature's head is located on a larger tentacle attached to the body, it is found directly above the location where the main body is underneath the ground. the creature can function without a head. The creature's actions are taken by the Maw, it has 120 hp of its own and when destroyed deals 50 damage to the body. the Maw may regrow at the beginning of its next turn if the Horror succeeds a Con Save DC15.

While the Maw is Destroyed the Body can be damaged by attacking the remaining tissue of the maw tentacle. 

Tentacle Attack - Melee weapon attack - 15ft reach +10 to hit 2d6+5 Bludgeoning damage
Attacks from any one tentacle at a target within reach of that tentacle

Slam - Horror Slams with all Tentacles, all creatures within 15ft radius of each tentacle must make DEX save DC17 or take 2d6 Bludgeoning damage and be knocked prone

Maw bite - Bites with Maw, One Target  25ft reach +10 to hit 3d8+5 Piercing
Target is knocked prone on hit.

Spines -  Horror throws a Volley of Spines from One Tentacle - one target 60ft range +5 to hit 3d4+5 piercing damage

If an enemy triggers an opportunity attack of a tentacle the horror can use it's reaction and choose to Tentacle attack the target or move the tentacle up to 15ft in any direction

The Horror can take 3 Legendary Actions, choosing from the options below, only one can be used at a time and only at the end of another creatures turn, unless otherwise stated.

Grasping Limbs (1 action) - The horror makes an attack with one of its tentacles at a target within range, on hit the tentacle may attempt to grapple the target

+10 to hit 15ft reach one target 2d6+5 damage

Regeneration (2 actions) : Regrow 1d4+1 tentacles below the ground, this cannot exceed the cap.

Grasp for Control (2 actions): Move one tentacle up to 60ft from its current location (cannot exceed 140ft from main body)

Enter the description for how mythic actions work for your monster here.

Mythic Action Name (Costs # Actions). Enter the description for your mythic action.