Magic Weapons. Aarkias’s weapon attacks are magical

Spellcasting. Aarkias is a 9th level spellcaster. Its spellcasting ability is intelligence (spell save DC 18, +8 to hit). Aarkias has the following warlock spells prepared:

Cantrips (at will): eldritch blast, light, mage hand, thorn whip

1st Level (4 slots): detect magic, mage armor, command, burning hands

2nd Level (3 slots): misty step, suggestion, agganazar’s scorcher

3rd Level (3 slots): elemental weapon, fireball, counterspell

4th Level (3 slots): greater invisibility, ice storm

5th Level (1 slot): cone of cold

Helm of Aarkeye. Aarkias has a helm that enables him to use truesight. As it is fused to his head, only extreme heal would melt it off, which would disfigure the horned devil. While wearing his helm, he has a perception of +11.

Multiattack. The devil makes three melee attacks

Fork. Melee Weapon Attack: +10 to hit, reach 10ft., one target.

Hit: 15 (2d8 + 6) piercing damage.

Tail.  Melee Weapon Attack:+ 10 to hit, reach 10ft., one target. Hit: 10 (1d8 + 6) piercing damage. If the target is a creature other than an undead or a construct, it must succeed on a DC 17 Constitution saving throw or lose 10 (3d6) hit points at the start of each of its turns due to an infernal wound. Each time the devil hits the wounded target with this attack, the damage dealt by the wound increases by 10 (3d6). Any creature can take an action to stanch the wound with a successful DC 12 Wisdom (Medicine) check. The wound also closes if the target receives magical healing. 

Hurl Flame. Ranged Spell Attack: +7 to hit, range 150 ft., one target. Hit: 14 (4d6) fire damage. If the target is a flammable object that isn't being worn or carried, it also catches fire. 

Aarkias can take two legendary actions, choosing from the options below. These effects are treated as bonus actions.

Teleport. Aarkias teleports to a unoccupied space within 120 ft of itself

Cast Spell. Aarkias casts a spell

Absorb Spell. Aarkias ignores a spell’s effects and is able to use it on his next turn without wasting a spell slot.