Famine's Desperation (recharges after a short/long rest). If Adam would be reduced to 0 hp, his current hp total instead resets to 425 hp, he recharges his Searing Heat/Corrosive Burn, and regains any expended uses of Legendary Resistance. Additionally, Adam can now use the options in the "Mythic Actions" section for 1 hour. Award a party an additional 105,000 xp (210,000 xp total) for defeating Adam after Famine's Desperation activates. 

Legendary Resistance (4/day). If Adam fails a saving throw, he can choose to succeed instead.

Unusual Nature. Adam doesn't require food or drink.

Multiattack. Adam makes one Bite attack and two Claw attacks

Bite. Melee Weapon Attack: +18 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage plus 13 (2d12) acid damage

Claw. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 19 (2d8 +10) slashing damage. If the target is a Huge or smaller creature, it is grappled (escape DC 20) and is restrained until this grapple ends. Adam can have only one creature grappled this way at one time. 

Tail. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 19 (2d8 +10) bludgeoning damage. If the target is a creature, it must succeed on a DC 26 Strength saving throw or be knocked prone. 

Searing Heat/Corrosive Burn (recharge 5-6). Adam releases a Breath Attack in a 300 ft. cone. Each creature in that area must make a DC 26 Dexterity saving throw. On a failed save, the creature takes 78 (12d12) fire (searing) or acid (corrosive) damage. On a successful save, the creature takes half as much damage. Only one version of the breath attack can be used at one time [i.e. searing heat, recharge, corrosive burn]. 

Adam can take 3 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. Adam regains spent legendary actions at the start of his turn.

Attack. Adam makes one Claw or Tail attack.

Wing Attack (Costs 2 Actions). Adam beats his wings. Each creature within 30 feet of it must succeed on a DC 26 Dexterity saving throw of take 17 (2d6 + 10) bludgeoning damage and be knocked prone. Adam can then fly up to half his flying speed. 

Piercing Hunger (Costs 3 Actions). Adam creates four spears of acid. Each spear hits a creature of Adam's choice he can see within 120 ft. of it, dealing 12 (1d8 + 8) acid damage to its target then disappears. Each creature targeted must succeed on a DC 26 Constitution saving throw or be inflicted with two points of exhaustion. 

Withering Spirit (Costs 3 Actions). Adam releases an orange fog. Each creature in a 25 ft. radius centered on him must succeed on a DC 26 Constitution save or be drained of 13 (3d8) hp. For each failure, Adam gains hp drained from each creature and gains half as much for each success. 

If Adam's Famine's Desperation trait has activated in the last hour, Adam can use the options below as legendary actions

Bite. Adam makes one Bite attack

Hunger Pangs (Costs 2 Actions). Adam screeches with dreadful energy. Each creature in a 60 ft. radius centered on him must succeed on a DC 26 Dexterity Saving throw or take 22 (5d8) acid damage. Each creature must succeed on a DC 26 Constitution saving throw or be inflicted with two points of exhaustion.