The undead spirit can be one of 3 types: Ghostly (30 HP, Flying speed 40 ft.), Putrid (30 HP), or Skeletal (20 HP). You add +3 health for each spell level above 3.
Festering Aura (Putrid Only). Constitution Saving Throw: DC equals your spell save DC, any creature (other than you) that starts its turn within a 5-foot Emanation originating from the spirit. Failure: The creature has the Poisoned condition until the start of its next turn.
Incorporeal Passage (Ghostly Only). The spirit can move through other creatures and objects as if they were Difficult Terrain. If it ends its turn inside an object, it is shunted to the nearest unoccupied space and takes 1d10 Force damage for every 5 feet traveled.
Multiattack. The spirit makes a number of attacks equal to half this spell's level (round down).
Deathly Touch (Ghostly Only). Melee Attack Roll: Bonus equals your spell attack modifier, reach 5 ft. Hit: 1d8 + 3 + the spell's level Necrotic damage, and the target has the Frightened condition until the end of its next turn.
Grave Bolt (Skeletal Only). Ranged Attack Roll: Bonus equals your spell attack modifier, range 150 ft. Hit: 2d4 + 3 + the spell's level Necrotic damage.
Rotting Claw (Putrid Only). Melee Attack Roll: Bonus equals your spell attack modifier, reach 5 ft. Hit: 1d6 + 3 + the spell's level Slashing damage. If the target has the Poisoned condition, it has the Paralyzed condition until the end of its next turn.
Transformation. The Balrog transformation lasts for 1 minute, after which the user falls unconsious for the following 8 hours.
Multiattack. As the balrog, make three attacks between: the tail, the horns, the breath weapon, the scythe, or the fire whip.
Tail. Melee Weapon Attack:+8 to hit, reach 5 ft., one target.
Hit: 6d6 + 7 bludgeoning damage.
Horns. Melee Weapon Attack:+8 to hit, reach 5 ft., one target.
Hit: 4d6 + 7 piercing damage.
Breath Weapon. The balrog exhales fire in a 60-foot cone. Each creature in that area must make a DC 18 Dexterity saving throw, taking 8d6 fire damage on a failed save, or half as much damage on a successful one.
Psionic Scythe. Melee Weapon Attack:+8 to hit, reach 20 ft.
Hit: 4d12 + 7 fire damage.
Flame Whip. Each creature in a 40-feet radius must make a DC 13 Dexterity saving throw, taking 3d12 slashing damage on a failed save, or half as much damage on a successful one.