Ancient Obsidian Dragon

Dragon Egg. When the dragon drops to 0 hit points, it radiates beams of purple light and then disappears, leaving behind a Medium jet-black dragon egg worth 5,000 gp. The DM rolls a d10 without showing the result. On a roll of 9 or less, the egg is inert. On a roll of 10, the egg will hatch into a chaotic neutral obsidian dragon wyrmling in 55 (22d4) days. The wyrmling will quickly imprint on a single creature who cares for it like a mother, if there is one, and after 11 (2d10) days of continued care, its alignment changes to match that of the creature it has imprinted on. Obsidian dragons grow faster than other dragons. The dragon is born Medium but grows to become Large after only 6 months, and Huge after 5 years.

For every hour a viable egg spends outside of the temperature range of 120-140 degress Fahrenheit, the DM rolls a d10 without showing the result. On a roll of 5 or less, the egg dies and becomes inert. On a roll of 6 or higher, it lives on. The egg has an AC of 15 and 20 hit points. It cannot regain hit points through natural means, only through magic, and is destroyed when reduced to 0 hit points. A living egg counts as a Medium dragon.

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

Gem Awakening (Recharges after a Short or Long Rest). If the dragon would be reduced to 0 hit points, its current hit point total instead resets to 350 hit points, it recharges its Breath Weapons, and it regains any expended uses of Legendary Resistance. Additionally, the dragon can now use its Mythic Action options during the next hour. Award the party exp for defeating two Ancient Obsidian Dragons if they defeat it after its Gem Awakening activates.

Multiattack. The dragon makes one Bite attack and one Claw attack and can use its Beating Wings.

Bite. Melee Weapon Attack: +15 to hit; reach 15 ft., one target. Hit: 18 (3d6 + 8) piercing damage plus 14 (4d6) necrotic damage.

Claw. Melee Weapon Attack: +15 to hit; reach 10 ft., one target. Hit: 17 (2d8 + 8) slashing damage. If the target is a Huge or smaller creature, it is grappled (escape DC 19) and is restrained until this grapple ends. The dragon can have only one creature grappled in this way at a time.

Beating Wings. The dragon beats its great wings. Each creature within 15 feet of the dragon must succeed on a DC 23 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage, be pushed 10 feet away from the dragon, and have the prone condition.

Breath Weapons (Recharge 5–6). The dragon uses its choice of one of the the following Breath Weapons:

Harmful Breath. The dragon exhales putrid gas in a 90-foot cone. Each creature in that area must make a DC 23 Constitution saving throw, taking 63 (18d6) necrotic damage on a failed save, or half as much damage on a successful one. The dragon then chooses a point it can see where the cone makes contact with the ground. That point becomes the center of a 15-foot-high, 15-foot-radius cylinder of lingering gas that disappears at the start of the dragon’s next turn. Any creature that starts its turn in the cylinder takes 14 (4d6) necrotic damage.

Magma Breath. The dragon exhales molten rock in a 90-foot cone. Each creature in that area must make a DC 23 Constitution saving throw, taking 66 (12d10) fire damage on a failed save, or half as much damage on a successful one. Objects within the cone that are not being worn or carried take 66 (12d10) fire damage and also ignite if they are flammable. The dragon then chooses a point it can see where the cone makes contact with the ground. That point becomes the center of a 15-foot-high, 15-foot-radius cylinder of lingering smoke that disappears at the start of the dragon’s next turn. The area within the smoke is heavily obscured

Spellcasting (Psionics). The dragon casts one of the following spells, requiring no spell components and using Charisma as the spellcasting ability (spell save DC 20):

At will: earth tremor (3rd-level version), hellish rebuke (3rd-level version)

1/day each: demiplane (see below), dispel magic, investiture of flame

Adult and Ancient Obsidian dragons typically use the Demiplane spell to access their lairs. If it casts the spell while already within its lair, it can extend the size of the lair in one dimension by 30 feet. Over the course of years they can create truly massive lairs.

Change Shape. The dragon magically transforms into any creature that is Medium or Small, while retaining its game statistics (other than its size). This transformation ends if the dragon is reduced to 0 hit points or uses a bonus action to end it.

Psychic Step. The dragon teleports to an unoccupied space it can see within 60 feet of it.

Hellish Rebuke. The dragon can cast hellish rebuke as a 3rd-level spell at will.

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

Claw. The dragon makes one Claw attack.

Psionics (Costs 2 Actions). The greatwyrm uses Psychic Step or Innate Spellcasting.

Psychic Beam (Costs 3 Actions). The greatwyrm emits a beam of psychic energy in a 90-foot line that is 10 feet wide. Each creature in that area must make a DC 20 Intelligence saving throw, taking 27 (5d10) psychic damage on a failed save, or half as much damage on a successful one.

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

Telekinetic Shove. The dragon can move one creature or object it has in its telekinetic grip from Mass Telekinesis up to 60 feet in any direction, but not beyond 120 feet of itself. In addition, it can choose to deal 45 (7d12) force damage to the creature or object if it wishes.

Mass Telekinesis (Costs 2 Actions). The dragon targets any number of creatures and objects it can see within 120 feet of it. No one target can weigh more than 4,000 pounds, and objects can’t be targeted if they’re being worn or carried. Each targeted creature must succeed on a DC 20 Strength saving throw or be restrained in the greatwyrm’s telekinetic grip. At the end of a creature’s turn, it can repeat the saving throw, ending the effect on itself on a success. If it uses this ability again, it releases all creatures and objects it was previously gripping. The dragon must concentrate to maintain this ability.