Aelthrys, The Black Sun

Black Sun's Hunger. Aelthrys is immune to all magic, absorbing the energy into himself. Instead of taking damage, he stores the full damage of any spell or magical effect directed at him. On his next unarmed strike, he releases this energy, dealing the stored amount as additional damage. 

- If the absorbed spell would apply a condition (hold person, blindness), Aelthrys is immune to the condition and instead adds 10 Force damage to his next unarmed strike. 

- If the spell is an AOE spell, he has advantage on the save, if he makes the save he can then absorb half the damage. Failing the save would result in him taking half damage. 

Eclipse Resonance. At the start of comabt, Aelthrys rolls 1d10 to determine which elemental damage type heals him instead of harming him. Any attack or effect that would deal this damage heals him for hald the damage it would have inflicted instead. He cannot absorb the damage type that currently heals him unless he is at full HP.

d10 Roll

1. Fire
2. Cold
3. Lightning
4. Thunder
5. Acid
6. Poison
7. Radiant
8. Necrotic
9. Force
10. Psychic

Darkened Flesh. Aelthrys is harmed by healing magic instead of restored by it. Whenever he is targeted by a healing effect, he takes damage equal to the healing amount instead. 

Void Aura. (Anti-Magic Field, 20ft) Magic falters in Aelthrys's presence. Spells of 3rd level or lower automatically fail within 20ft of him. Higher-level spells require a DC 18 spellcasting check or fail. Magic items lose all enchantments while in this aura.

Mundane Vulnerability. Aelthrys is vulnerable to damage dealt to him by non-Magical weapons

Multiattack. Aelthrys makes three melee attacks

Umbra, the Black Sun. Greatsword, +14 to hit, reach 10ft. Hit: 26 (4d6+8) slashing damage + 14 (4d6) Force damage. If the target is a spellcaster, they lose a spell slot of the highest level available as the blade devours their magic.

Abyssal Strike. Unarmed, +14 to hit, reach 5ft. Hit: 20 (3d6+8) Bludgeoning damage. If Aelthrys has absorbed a spell, he unleashed the full stored damage of the spell on his first strike.

Void Pulse. (Recharge 5-6) Aelthrys releases a wave of anti-magic energy in a 30ft. radius. Any creature in the area must make a DC 20 CON save, taking 10d8 Force damage on a fail, or half as much on a success. Magical effects and summoned creatures within the area are immediately dispelled or destroyed.

Aelthrys can take up to 3 Legendary Actions, choosing from the options below. Only one can be used at a time and only at the end of another creature's turn. 

Void Step. (1 Action) Aelthrys teleports up to 40 feet to an unoccupied space he can see. 

Eclipse Rend. (1 Action) Aelthrys makes one Abyssal Strike. If he has stored spell damage, he can choose to split it between two creatures within range. 

Black Sun Awakens. (2 Actions) Aelthrys' next attack absorbs a second spell (if one is cast before his next turn). If a spell is absorbed this way, he immediately takes another action.