Black Greatwyrm (Modified)

Amphibious. The greatwyrm can breathe air and water.

Chromatic Awakening (Recharges after a Short or Long Rest). If the greatwyrm would be reduced to 0 hit points, its current hit point total instead resets to full hit points, it recharges its Breath Weapon, and it regains any expended uses of Legendary Resistance. Additionally, the greatwyrm can now use the options in the “Mythic Actions” section for 1 hour. Award a party an additional 75,000 XP (150,000 XP total) for defeating the greatwyrm after its Chromatic Awakening activates.

Legendary Resistance (4/Day). If the greatwyrm fails a saving throw, it can choose to succeed instead.

Magic Weapons. The greatwyrm’s weapon attacks are magical.

Unusual Nature. The greatwyrm doesn’t require food or drink.

Multiattack. The greatwyrm can use it's frightful presence. The greatwyrm then makes three attacks: one with it's Bite attack and two with it's Claw attacks.

Bite. Melee Weapon Attack: +18 to hit, reach 15 ft., one target. Hit: 23 (2d12 + 10) piercing damage plus 16 (3d8) acid damage, and if the target is a creature it must succeed on a DC 20 Dexterity saving throw or it takes an additional (2d8) damage at the start of its next turn. 

Claw. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 23 (3d8 + 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. The greatwyrm can have only one creature grappled this way at a time.

Tail. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 23 (3d8 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 26 Strength saving throw or be knocked prone.

Frightful Presence. Each creature of the greatwyrm's choice that is within 450 feet of the greatwyrm and aware of it must succeed on a DC 23 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the greatwyrm's Frightful Presence for the next 24 hours.

Acid Breath (Recharge 5–6). The greatwyrm exhales a deathly stream of black acid in a 450-foot line that is 15 feet wide. Or it selects two points within 450 feet of both each other and itself, with a 15 foot wide line connecting those two points. Regardless of the choice, each creature in the area of that line must make a DC 24 Dexterity saving throw. On a failed save, the creature takes 99 (22d8) acid damage. On a successful save, the creature takes half as much damage.

The greatwyrm 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. The greatwyrm regains spent legendary actions at the start of its turn.

Attack. The greatwyrm makes one Claw or Tail attack.

Wing Attack (Costs 2 Actions). The greatwyrm beats its wings. Each creature within 30 feet of it must succeed on a DC 26 Dexterity saving throw or take 17 (5d6) bludgeoning damage and be knocked prone. The greatwyrm can then fly up to half its flying speed.

Arcane Spear (Costs 3 Actions). The greatwyrm creates four spears of magical force. Each spear hits a creature of the greatwyrm’s choice it can see within 300 feet of it, dealing 11 (1d8 + 7) force damage to its target, then disappears.

If the greatwyrm’s Chromatic Awakening trait has activated in the last hour, it can use the options below as legendary actions.

Bite. The greatwyrm makes one Bite attack.

Acidic Eruption (Costs 2 Actions). The greatwyrm releases a violent eruption of deadly black acid from its scales. Each creature in a 90-foot-radius sphere centered on the greatwyrm must succeed on a DC 24 Dexterity saving throw or take 22 (5d8) acid damage.