Reactive Immunity: When it takes [necrotic] or [radiant] damage, the heads can switch their immunity to either of those two damage types. If it is immune to necrotic damage at the time, it can use a reaction to switch its immunity to radiant if necessary.

Can only be used (4 times per long rest).

Spellcasting. The [Abeloth] is a 6-level spellcaster. Its spellcasting ability is [charisma] (spell save DC 19, +10 to hit with spell attacks). The [Abeloth] has following [wizard], [warlock] and [sorcerer] spells prepared:
Cantrips (at will): [Minor Illusion], [Mold Earth] 
3rd level (3 slots): [Daylight], [Darkness], [Crown of Madness], [Counterspell]

Multiattack. The [Abeloth] makes one bite attack with each of its heads.

Bite. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) [piercing] damage, plus 13 (2d12) [force] damage.

Claw. Melee Weapon Attack: +10 to hit, range 5/15 ft., one target. Hit: 19 (2d8 + 10) [slashing] damage. If the target is huge, or smaller they are  grappled and restrained (escape DC 18 STR). 

Tail. Melee Weapon Attack: +10 to hit, reach 20ft., two targets. Hit: 19 (2d8 + 10) [bludgeoning] damage. If the targets are creatures they must succeed on a DC 20 STR saving throw, or be knocked prone.

Breath Weapon (Recharge 5-6). The [Abeloth] exhales a blast of energy in a 300-foot cone. Each creature in that area must make a DC 20 DEX saving throw. On a failed save, the creature takes 10d10 [radiant] damage. On a successful save, the creature takes half as much damage.

[Optional]. Smouldering Glare. Seduces the bard., Hit: 6 (2d4) [psychic] damage.

Opportunity. Opportunity Attack.

Spell: 3rd level: [Counterspell]

The [Abeloth] can take 4 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The [Abeloth] regains spent legendary actions at the start of its turn.

Billowing Wings (Costs 1 Action). The [Abeloth] beats its wings. Each creature within 60ft must make a DC 17 STR saving throw, on a fail the creatures are knocked prone, on a success, the creatures are move backwards 10ft.

Radiant Spear (Costs 3 Actions).  The White Head of the [Abeloth] creates four spears of magical force. Each spear hits a creature of the [Abeloth] choice it can see within 120 feet of it, dealing  (1d8 + 16) [force] damage to its target.

Ruinous Wake (Costs 3 Actions). The Black Head of the [Abeloth] spews a beam of black fire in a 10ft wide and 300ft straight line that lays across the ground. At the end of the next person's round, multiple small explosions of black sphere's occur and each creature within 20ft must make a DC 17 DEX saving throw. You take (2d8 + 16) [necrotic] damage on a failure, half as much on a success. Once used the [Abeloth] falls to the ground and becomes vulnerable to all damage (excluding it's immunities) for 1 round.

If the [Abeloth] drops below 300hp, the mythic actions kick in; they cost 3 actions and only one mythic action can be made per long rest.

Severance (Costs 3 Actions). The [Abeloth] utters a chant in an unknown language and tears the fabric of the realms, cutting off all use to magic (spells) until the end of the initiative order, by which time the tear will repair itself and the use of magic (spells) will resume.

Teraflare (Costs 3 Actions). Both heads lurch up and use their breath weapons to create a sphere of radiant and necrotic light, which is 60ft wide, centred directly in front of it. Any creature within the sphere takes (3d12) [radiant] damage and (3d12) [necrotic] damage and has to take the dash action on their next turn to get out of it.