Amphibious. This god can breathe air and water.

If anyone or deity would mess with one of his curses, he would then teleport to the area to deal with the being in question, using their magic to teleport them to him, as he would then make sure his son/daughter, the curse, was alright, curing any curses or illnesses that would mess or interfere with his curse

immunity to acid and posion

Legendary Resistance (3/Day, or 4/Day in Lair). If the dragon fails a saving throw, it can choose to succeed instead.

Lair action on a roll of 19 or 20 abbathor teleports the party to his lair, which his filled with poison gas and smoke, which the party must make a con save and wisdom save or be poisoned and cursed(have disadvantage on all atk rolls till the end of combat. 

Lair action up to once per turn as a reaction, Abbathor can become one with the poison gas/smoke in his lair to become intangible to all damage except for fire and lightning damage

Multiattack. This god can make three melee attacks. It can replace up to two attacks with a use of Spellcasting to cast [spellsCircle of Death[/spells] (level 7 version).

smash. Melee Attack Roll: +11, reach 10 ft. Hit: 13 (2d6 + 6) Slashing damage plus 4 (1d8) Acid damage.

Acid Breath (Recharge 5–6). Dexterity Saving Throw: DC 20, each creature in an 80-foot-long, 10-foot-wide Line. Failure: 54 (12d8) Acid damage. Success: Half damage.

Spellcasting. The god casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 17, +9 to hit with spell attacks):

At Will: Counterspell, Fear, Circle of Death (level 3 version)

1/Day Each: Prismatic Spray, Vitriolic Sphere

As a bonus action Abbathor can cast Blade of Disaster

Lair action up to once per turn as a reaction, Abbathor can become one with the poison gas/smoke in his lair to become intangible to all damage except for fire and lightning damage

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature's turn, the god can expend a use to take one of the following actions. The god regains 2 expended uses at the start of each its turns.

Cloud of poison. Dexterity Saving Throw: DC 17, one creature the god can see within 120 feet. Failure: 22 (4d10) Poison damage, and the target has Disadvantage on saving throws to maintain Concentration until the end of its next turn. Failure or Success: The dragon can’t take this action again until the start of its next turn.

Poisonous Presence. The god uses Spellcasting to cast [Tooltip Not Found]. The god can’t take this action again until the start of its next turn.

smash. The god can move up to half its Speed, and it makes one sweeping attack with his battle axe.

Upon going to zero hit points, he regains all hit points and transforms in a ancient black dragon size become huge and can't take damage till the start of his next turn takes a mythic action to cause one of the following effects. The god cannot cause the same effect twice in a row.

Essence of acid. The god becomes one with the acid until the his next turn. When a creature within 15 feet of him hits him with a melee attack, it must succeed on a DC 20 Constitution saving throw or be restrained until the start of its next turn it also takes 6d8 acid damage.

Spirit of poison. The god is surrounded by a cacophonous burst of poison in a 120-foot-radius sphere centered on him until the next round. Enemy creatures in the affected area are poisoned and take 12d8 poison damage. Also, the god gains the invisible condition till his next turn. Additionally, when an enemy creature in the area casts a spell with a verbal component, it must succeed on a DC 20 Constitution saving throw or the spell fails, and any resources used to attempt the cast are expended.