Flyby. The undead angel doesn’t provoke an opportunity attack when it flies out of an enemy’s reach.

Innate Spellcasting. The undead angel’s innate spellcasting ability is Charisma (spell save DC 18, +10 to hit with spell attacks). The angel can innately cast the following spells, requiring no material components:

3/day each: cause fear, acid arrow (as a 5th-level spell)

1/day each: false life, fireball (as a 6th-level spell)

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

Relentless Dead (Recharges after a Short or Long Rest). If the undead angel takes 21 damage or less that would reduce it to 0 hit points, it is reduced to 1 hit point instead.

Multiattack. The angel makes two melee attacks.

Longsword. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 10 (1d8 + 6) slashing damage, or 11 (1d10 + 6) slashing damage if used with two hands, plus 22 (5d8) fire or necrotic damage (undead angel's choice).