Arkhao Thornas, the Corrupted One

Corrupted Form. In his normal form, Arkhao appears to be a skinny elven male in his 30's having dark long hair and a short unkempt beard. Whenever Arkhao takes 10 hit points of damage from anything, Arkhao must make a DC 20 Constitution saving throw. If it fails, Arkhao is enveloped by growing bluish slimy vines growing out of his body which then after a few seconds turns into dark blue steel like material; he's now in Corrupted Form. When he is in corrupted form, he becomes more muscular and becomes 10 feet in height, his personality changes drastically toward Chaotic Evil.

Legendary Resistance (3/Day). If Arkhao fails a saving throw, he can choose to succeed instead.

Regeneration. At the end of each of Arkhao's turn, he regenerates 1d20 hit points or half as much if the took acid or fire damage during the round.

Bite. Melee Weapon Attack: +16 to hit, reach 5 ft., one target. Hit: 10 (1d10 + 8) piercing damage.

Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 16 (2d8 + 8) slashing damage.

Thunderous Whip. One of the tentacles of Arkhao whips quickly toward a target within 15 ft. dealing 2d6 + 8 slashing damage plus 1d8 thunder damage to any creature within 5 ft. sphere of the impact. 

Frightful Presence. Each creature of the Arkhaos choice that is within 120 feet of him and aware of it must succeed on a DC 16 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the Arkhao's Frightful Presence for the next 24 hours.

Grapple. Arkhao lunges toward a target within 5 feet of him and grapples the target with his metallic appendages. Target must make a DC 20 Dexterity saving throw, becoming grappled on a failed save. The grappled creature must make a DC 20 Strength check to escape. Arkhao cannot move when this ability is active.

Reinforce Body. Whenever an attack would hit Arkhao, Arkhao's body becomes harder for the impact, increasing his AC by 8 for the first attack that hit him during this turn.

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

Detect. Arkhao makes a Wisdom (Perception) check.

Devour (Costs 2 Actions). If a target is grappled by Arkhao, Bite is used three times against the grappled target.