Akilla the Colossal Skeleton

Steps of Beyond: Akilla can at will enter the Ethereal Plane from the Material Plane, or vice versa. It is impossible to detect in any way other than ethereal sight on the Material Plane while it is in the Border Ethereal, and vice versa, yet it can't affect or be affected by anything on the other plane.

Ethereal Sight: Akilla can see up to 300ft into the Ethereal plane while it is on the Material plane and vice versa.

Necrotic Absorption: Whenever Akilla would take necrotic damage it regains hit point equal to the damage instead.

Magic Resistance. The Akilla is immune to spells cast with spell slots of 6th level or lower and has advantage on saving throws against spells and other magical effects.

Immutable Form. Akilla is immune to any spell or effect that would alter its form. 

Colossal Size: Akilla is colossal, it ocupies a 40ft wide 40ft long 140ft tall space.

Weakness to Holy Light: Akilla is weak agaisnt radiant damage, whenever it takes radiant damage it cannot use its Steps of Beyond trait, and takes an additional +20 radiant damage.

Magic Weapons. Akilla´s weapon attacks are magical.

Moving Mountain: Akilla is immune to any speed reduction other than from spells cast with spell slots of 6th level or higher.

Seige Monster: Akilla deals double damage to structures.

Multiattack: Akilla can make up to two melee weapon attacks with his attack action.

Giant Hands: Akilla has huge skeletal hands:
Spear of Death: Melee Weapon Attack +15 to hit, 35ft reach, one target, for 6d12+7 piercing damage a creature within 5ft of the target must succeed in a DC 23 Dexterity saving throw taking half of the damage taken by the target on a failure.
Colossal Grasp: All creatures within a 10ft cube within 35ft of Akila must succeed in a DC 23 Dexterity saving throw, on a failure they take 6d10+6 bludgeoning damage and are restrained by the hand of Akilla, a creature can as an action atempt to succeed in a DC 23 Strenght saving throw to escape the restraining hand, tiny and gargantuan creatures cannot be restrain by Akilla´s graps, however they still take damage.
All creatures hit by the attack that can be restrained by it and are within the targeted area are restrained in one of Akilla´s hands.
The Akilla can only perform this attack if it has at least one free hand.

Stomping Feet: Akilla is colossal and has enormous skeleton feet:
Crushing Stomp: Melee Weapon Attack +15 to hit, All creatures within a 10ft cube within 20ft of Akilla, for 6d12+7 bludgeoning damage, a creature hit by this attack must succeed in a DC 23 Strenght saving throw, falling prone on a failure.
Stomping Frenzy (Recharge 5-6): All creatures within a 40ft radius centered on Akilla must succeed in a DC 23 Dexterity or Strenght saving throw falling prone and taking 12d12+12 bludgeoning damage on a failure, half as much damage on a success and no additional effect.

Prone creatures have disadvantage on this saving throw.

Negative Energy Wave (Recharge 5-6): All creatures within 120ft of Akilla (Akilla included) must succeed in a DC 23 Constitution saving throw as they suport the decay and necrotic energies, taking 10d10+10 necrotic damage on a failre, half as much on a success. 

Crush: Akilla can crush all creatures and objects restrained by its Giant Hands dealing 6d12 bludgeoning damage to all creatures and stroctures restrained in this way.