Balor Thing of Slime

Corrosive Form.

 A creature that touches the balor slime or hits it with a melee attack while within 5 feet of it takes 7 (1d12) acid damage. Any nonmagical weapon made of metal or wood that hits the balor slime corrodes. After dealing damage, the weapon takes a permanent and cumulative -1 penalty to damage rolls. If its penalty drops to -5, the weapon is destroyed. Nonmagical ammunition made of metal or wood that hits the balor slime is destroyed after dealing damage.

Toxic Aura.

 At the start of each of the balor slime's turns, each creature (that relies on breathing) within 5 feet of it takes 10 (3d6) poison damage, and must succeed on a DC 16 CON Saving Throw or become poisoned.

Death Throes.

 When the balor slime dies, it explodes, and each creature within 30 feet of it must make a DC 19 Dexterity saving throw, taking 20 (5d8) acid damage on a failed save, or half as much damage on a successful one. Each creature (that relies on breathing) within 60 feet takes 30 (10d6) poison damage, and must succeed on a DC 20 Constitution saving throw or become poisoned, taking only half damage on a success and resisting the poisoned condition. The toxic fumes linger for 1 hour and heavily obscure the area.

Magic Resistance.

 The balor slime has advantage on saving throws against spells and other magical effects.

Magic Weapons.

 The balor slime's weapon attacks are magical.

Cold Susceptibility.

 Cold damage against the balor slime reduces it's speed by 5ft. until the end if its next turn. If the balor slime's speed is reduced to 0 feet in this way, it is stunned until the end of its next turn.

Water Susceptibility.

 While in at least 20 ft.(or bombarded by at least 30 gallons) of water, the balor slime has a -2 penalty to all attacks, ability checks, and saving throws.

Flame Sustainability.

 Fire damage instead heals the balor slime. If this healing effect would exceed the balor slime's maximum hit points by at least 40 points, the balor slime transforms into a Balor.

Multi-Attack.

 The balor slime makes two attacks: one with its longsword and one with its claw

Claw.

 Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 16 (2d8 + 7) slashing damage.

Longsword.

 Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 20 (3d8 + 7) slashing damage plus a 50% chance of 13 (3d8) lightning damage.

Engulf.

 The balor slime moves up to its speed. While doing so, it can enter Large or smaller creatures' spaces.

Whenever the balor slime enters a creature's space, the creature must make a DC 15 Dexterity saving throw. On a successful save, the creature can choose to be pushed 5 feet back or to the side of the balor slime. A creature that chooses not to be pushed suffers the consequences of a failed saving throw.

Teleport.

 The balor slime magically teleports, along with any equipment it is wearing or carrying, up to 120 feet to an unoccupied space it can see. The balor slime cannot teleport into or out of water