Ahriman

Limited Magic Immunity. Ahriman is immune to effects from 7th level spells or lower. he has advantage on saving throws for all other magical effects.

Trapped and Wounded. The primordial serpent is currently trapped and wounded in the serpent's coils in Nessus, Baator. He must devour enough souls to heal and regain his former glory.

Wish resistance. When someone casts a wish to restrain Ahriman or wish him away or affect him in any way, he gets to make a Charisma saving throw.

 Sanctuary. Ahriman is permenently under the effects of a sanctuary spell.

Legendary resistance (5/day). when Ahriman fails a saving throw he can chose to succeed instead. 

Multiattack. Ahriman uses his Enchanting Gaze and makes two attacks, once with his bite and once with a headbut. The bite can be replaced with a swallow and is usually used last.

Bite. Melee Weapon Attack: +20 to hit, reach 10 ft., one target. Hit: 36 (4d12 + 10) piercing damage. If the target is a creature, it is grappled (escape DC 30). Until this grapple ends, the target is restrained, and Ahriman can't bite another target.

Headbut. Melee Weapon Attack: +20 to hit, reach 10ft., one target. Hit: 42 (5d12+10) bludgeoning damage. The target must make a DC 30 strength saving throw or be knocked prone.

Swallow. Ahriman makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the target takes the bite's damage, the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the tarrasque, and it takes 56 (16d6) acid damage at the start of each of Ahriman's turns.

If Ahriman takes 60 damage or more on a single turn from a creature inside it, he must succeed on a DC 30 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the his mouth. If the Ahriman dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Enchanting Gaze (recharge 5-6). Ahriman looks someone in the eye and charms them somehow. the target must make a DC 30 wisdom saving throw or be charmed by Ahriman. he can make another saving throw at the end of his turn. A creature charmed by Ahriman takes every suggestion as if it where a suggestion.

Summon Asmodeus. If Asmodeus is still 'alive' or has been dead for more than month, Ahriman can summon him to his side. As Asmodeus is actually an avatar of Ahriman, Ahriman can't take and actions, reactions, bonus actions or legendary actions if Asmodeus does something. they take the same turn in initiative.

Counterspell (3/day). Ahriman can cast counterspell at 9th level.

Ahriman has 5 legendary actions. he can use these at the end of another creature's turn, but only one at a time.

Roar (costs 2 actions). Everyone within 500ft makes a DC 30 wisdom saving throw or be frightened until the end of Ahriman's next turn. People frightened in this way are stunned until the end of Ahriman's next turn.

Rumble. Ahriman moves and the earth and cliffs around him rumble. everyone within 30ft of him must make a DC 20 Dexterity saving throw or take 25(5d6+5) bludgeoning damage as rocks fall down.

Nudge. Ahriman nudges the people closest to him. everyone within 10 ft must make a DC 30 strength saving throw or fall prone.

Create fiend (costs 3 actions). Ahriman creates a devil of Challenge rating 2 or higher that joins the battle on his side, incapable of hurting or even disobeying Ahriman. This fiend is rolled randomly and due to the mind controll it will be immune to enchantment.