Amon

Cataclysmic End. When the Amon's hit points are reduced to 0, he 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 Amon’s weapons. This then triggers Amon's "Soulless Immortality" trait.

Fire Aura. At the start of each of Amon’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 Amon or hits it with a melee attack while within 5 feet of it takes 10 (3d6) fire damage.

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

Magic Weapons. Amon's weapon attacks are magical.

Soul's for Avernus: Amon's power is fueled by the fire's of Avernus. Souls collected by Amon are sent directly to Avernus, he is immediately rewarded with greater power. When a creature is killed by Amon, he receives half of the creature's hit point total in healing. Amon and his "Fire Aura" also increase 5ft in size. If Amon receives more than 50hp in healing in one turn, wing burst from his back, doubling his flying speed and dripping molten lava. Any space that Amon moves over is covered in lava that deals 4d6 fire damage to any creature it touches.

Soulless Immortality. Amon cannot die. After "Cataclysmic End" resolves, an infernal glowing stone is left in the place of Amon's body. Trapped inside is Amon's soulless essence. The stone cannot be destroyed by any means short of a "Wish" spell, or by tossing it in the fires in which it was forged.

Multiattack. Amon makes two attacks: one with his longsword and one with his Fire Chains.

Longsword. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 21 (3d8 + 8) slashing damage plus 13 (3d8) fire damage. If Amon scores a critical hit, it rolls damage dice three times, instead of twice.

Fire Chains. Melee Weapon Attack: +14 to hit, reach 30 ft., one target. Hit: 15 (2d6 + 8) slashing damage plus 10 (3d6) fire damage, and the target must succeed on a DC 20 Strength saving throw or be pulled up to 25 feet toward Amon.

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

Hellfire Orb. Amon magically casts a flame, up to 80 feet, that explodes on contact into a 30 foot sphere dealing 6d6+12 fire damage. OR Amon can cast 4 individual blast dealing 1d6+3 fire damage, each blast can be targeted individually.

Amon can only take legendary actions while in his true form.
Amon can spend a legendary action at the end of any turn.

Demon Prince's fury
Amon can spend a legendary action to have a full turn, he is able to move, cast spells or attack. Amon's passive ability's do not trigger or resolve on legendary actions.

Legendary Resistance
If Amon fails a saving throw, he can spend a legendary action and choose to succeed instead.