Crota, Voice of the Thousands

Magic Resistance. Crota has advantage on saving throws against spells and other magical effects.

Spider Climb. Crota can climb difficult surfaces, including upside down on ceiling, without needing to make an ability check

Corrosive Aura. Any creature beginning its turn within 15 feet of Crota takes 10 acid damage

Innate Spellcasting. Crota's spellcasting ability is Intelligence (Spell save DC 19, +11 to hit with spell attacks). Crota can innately cast the following spells, requiring no material components: 

At will: Acid Splash (17th level), detect magic
1/day each: blight, contagion, gaseous form

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

Grasping Tendrils. Crota can have up to 3 tendrils at a time. Each Tendril can be attacked (AC 20; 15 hit points; immunity to poison and psychic damage). Destroying a tendril deals no damage to Crota, who can extrude a replacement tendril on its next turn. A tendril can also be broken if a creature takes an action and succeeds on a DC 18 Strength check against it.

Multiattack. Crota can take as many attacks as he has tendrils, one of these attacks.

Piercing Tendril. Melee Weapon Attack: +14 to hit, reach 30 ft., 1 target. Hit:  17 (3d6 + 8)  piercing damage. If Crota hits, the target is impaled and pulled up to 10 feet towards Crota, the target can make a Strength check DC 14 to free themselves. Any creature  drawn into Crota's body takes 30 acid damage at the beginning of Crota's turn. A creature reduced to 0 hit points inside of Crota has their corpse obliterated in one minute.

Bludgeoning Tendril. Melee Weapon Attack: +14 to hit, reach 20 ft., 1 target. Hit:  20 (4d6 + 8) bludgeoning damage. Wherever Crota impacts, any creature within 5 feet on the impact zone must make a DC 18 strength saving throw or take half the damage dealt by this attack and fall prone. This attack deals double damage to structures and objects.

Slashing Tendril. Melee Weapon Attack:  +14 to hit, reach 10 ft., 1 target. Hit: 23 (5d6 + 8) slashing damage. The creature hit by this attack must make a DC 14 Constitution saving throw or take half the damage dealt by this attack at the beginning of their next turn.

Oversoul Barrage (Recharge 6). Crota summons his oversoul above him, which grows to triple its original size. Each creature within 120 feet of Crota and not behind total cover must make a DC 22 strength saving throw or take 40 (10d8) Necrotic damage on a failed save, or half as much on a successful one.

Crota can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time, and only at the end of another creature's turn. Crota regains spent legendary action at the start of its turn.

Tendril. Crota can make a piercing or bludgeoning tendril attack.
Acid Splash. Crota casts Acid Splash.
Corrupting Touch (Costs 2 actions). Melee weapon attack: +14 to hit, reach 10 ft., one creature. Hit: 14 (2d6 + 8) acid damage, and the target is slimed. Until the slime is scraped off as an action, the target is poisoned, and any creature, other than an abberation, while within 10 feet of the target is also poisoned.