Telepathic Command. Once the fiend sends a creature into madness with Touch of Madness, he is able to command the creature to harm themselves. The target must make a DC15 Wisdom saving throw or take 2d8+2 damage as they cut into themselves with their weapon or their own bare hands.

Fire Shield. Whenever a creature that is within 10ft of Zeriarth hits him with a melee attack, the attacker immediately takes 2d8 fire damage.

Spellcasting. Zeriarth is a 10th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 17, +9 to hit with spell attacks). Zeriarth has following Warlock spells prepared:
Cantrips (at will): Eldritch Blast, Chill Touch
1st level (# slots): Cause Fear, Hellish Rebuke

2nd level (# slots):Blindness/Deafness, Darkness

3rd level (# slots):Fireball

4th level (# slots):Dimension Door

5th level (# slots):Synaptic Static, Danse Macabre, 

Claw (Fiend Form Only). Melee Weapon Attack: +5 to hit, reach 5ft., one target. Hit: 7 (1d10 + 3) slashing damage.

Touch of Madness. Melee Weapon Attack: +5 to hit, reach 5ft., one target. Hit: A dark image appears in your mind, tearing your will asunder. The affected creature must make a DC15 Wisdom saving throw. On a failed save, the creature takes 4d8 + 5 psychic damage and is frightened until the next turn. On a successful save, the creature takes half damage and is not frightened.

Draining Kiss. The fiend kisses a creature frightened by it or a willing creature. The target must make a DC 15 Constitution saving throw against this magic, taking 32 (5d10 + 5) psychic damage on a failed save, or half as much damage on a successful one. The target's hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.

Zeriarth can take 3 legendary actions. Only one legendary option can be used at a time and only at the end of another creature's turn. The legendary monster regains spent legendary actions at the start of its turn.

Additional Attack (Costs 1 Action). At the end of another creature's turn, Zeriarth can attempt to use the Draining Kiss or Touch of Madness action against any creature that is frightened or willing and within range.

Cast a Spell (Costs 3 Actions). At the end of another creature's turn, Zeriarth can cast a spell as a legendary action.

Legendary Resistance (3/day). If Zeriarth fails a saving throw, it can choose to succeed instead.