Death Throes. When the zothtor dies, it explodes, and each creature within 30 feet of it must make a DC 20 Dexterity saving throw, taking 70 (20d6);fire damage on a failed save, or half as much damage on a successful one. The explosion ignites flammable objects in that area that aren’t being worn or carried, and it destroys the zothtor's weapons.

Fire Aura. At the start of each of the zothtor's turns, each creature within 5 feet of it takes 10(3d6);fire damage, and flammable objects in the aura that aren’t being worn or carried ignite. A creature that touches the zothtor or hits it with a melee attack while within 5 feet of it takes 10 (3d6);fire damage.

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

spell casting:

Fire Storm: a storm made up of sheets of roaring flame appears in a location zothtor  chooses within range. the area of the storm consists of up to ten 10-foot cubes, which zothtor can arange as zothtor wishes. each cube must have at least one face adjacent to the face of another cube. each creature in the area must make a dexterity saving throw. it takes 7d10 fire damage on a failed save, or half as much danage on a successful one.

Death's Door:

Deathly Blade: the zothtor has a +2 bonus to attack and damage rolls made with this magical weapon and creatures that are hit by it suffer an extra 2d6 slashing damage

Deaths call: zothtor can mark an enemy for death. as an action, zothtor can expend 2 charges so a creature zothtor can see within 30ft. makes a DC 15 constitution saving throw, suffering 5d6 necrotic damage and being marked for death for 1 min. on a failure, but only suffering half as much damage on a success. a creature marked for death has their AC reduced by 2 and makes the first saving throw they make each round with disadvantage. they repeat constitution saving throw at the start of each of their turns, removing the mark on a success. zothtor can mark a creature for death expending only 1 charge when zothtor scores a critical hit on them with deaths door.

Planar Rip: deaths door is made up of a rip in the fabric of existence. zothtor can temper with it to cast destructive wave(3 charges, DC 15), result of its unstable nature. if a creature that's originally from another plane fails the saving throw against the spell, zothtor can expend 2 additional charges to send them back to there native plane, as if affected by banishment.

Unstoppable Force: death cant be stopped forever. zothtor embodies a certain measure of this aspect: zothtor can expend 1 charge as a reaction to making a ability check, attack roll or saving throw with disadvantage to roll normally instead. zothtor can also expend 1 charge to stop an effect from moving zothtor against your will or reduce your speed.

Teleport. The balor magically teleports, along with any equipment it is wearing or carrying, up to 120 feet to an unoccupied space it can see.