Ancient Shadow Obsidian Dragon

Legendary Resistance (5/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Ancient Fortitude. The dragon is immune to any spell or effect that would alter it's form, move it against its will, or move it to another plane of existence against its will.

Living Shadow. While in dim light or darkness, the dragon is resistant to all damage types except force and radiant.

Shadow Stealth. While in dim light or darkness, the dragon can Hide as a bonus action.

Sunlight Sensitivity. While in sunlight, the dragon has disadvantage on all attack rolls, as well as Perception checks that rely on sight.

Volcanic Heat. The Shadow Obsidian Dragon exudes overwhelming heat that causes the area around it to crackle and sizzle with sparks and fire. The area within 15 ft of the dragon is difficult terrain. Plant life burns instantly within this radius. Additionally, Fire and Necrotic damage dealt by the dragon ignores resistances to fire and necrotic damage, and treats immunity as resistance. If the dragon takes cold damage, this ability no longer functions until the end of its next turn, or until it takes fire damage or enters difficult terrain the dragon has created.

Lava Born. Difficult terrain created by the dragon becomes a volcanic hellscape, sapping the strength from any foolish enough to remain in it, and causing any wooden structures to spontaneously combust. When a creature starts its turn in difficult terrain created by the dragon, or enters it for the first time in a round, they take 3 (1d6) fire and 3 (1d6) necrotic damage. Furthermore, they must succeed a DC 22 Constitution saving throw, or suffer from disadvanatge on all ability checks and saving throws for 1 minute. The creature can reroll the saving throw as an action, ending the effect and becoming immune to it for 1 hour on a successful save. Any creatures with resistance to fire or necrotic damage, or have any other benefit against extreme heat or exhaustion, automatically succeed this saving throw.

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +18 to hit, reach 15 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 14 (4d6) necrotic damage.

Claw. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8) slashing damage.

Tail. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 17 (2d8 + 8) bludgeoning damage. A creature struck by this attack must also make a DC 16 Dexterity saving throw, or be cut by the sharp obsidian scales covering its tail, taking an additional 9 (2d8) slashing damage, and takes an additional 2 (1d4) slashing damage at the end of its next 5 turns, as it sufferes from bloodloss from the razor-edged scales. This can be ended by making a DC 15 Constitution check, or by receiving any form of healing. On a successful save, the creature takes no additional slashing or bloodloss damage, able to avoid the sharper edges of the dragon's scales.

Frightful Presence. Each creature of the dragon’s choice that is within 120 feet of the dragon and aware of it must succeed on a DC 22 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 dragon’s Frightful Presence for the next 24 hours.

Molten Shadow Breath (Recharge 5–6). The dragon exhales molten rock and shadow flames in a 90-foot cone. Each creature in that area must make a DC 25 Dexterity saving throw, taking 42 (12d6) bludgeoning damage and 42 (12d6) necrotic damage on a failed save. Additionally, the creature is restrained by newly formed obsidian chunks and volcanic rocks until it uses its action to escape, required a DC 18 Strength saving throw, or until another creature uses its action to free the restrained one, requiring no save. If a creature succeeds on the original saving throw, it only takes half damage, and is not restrained. Any creatures reduced to 0 hitpoints by this attack dies, and a Shadow rises from its corpse under the dragon's command. Furthermore, the area affected by the breath weapon becomes difficult terrain for 10 minutes.

Genesis (1/day). The dragon concentrates all of its Psionic power to create a plane of its own design. Every creature within 300 ft must succeed a DC 25 Charisma saving throw, or be teleported into a demiplane of the dragon's own design. The exact layout is determined by the dragon, but will almost always consist of volcanic terrain, lava, smoke and rock, with an approximate radius of 120 ft. When the dragon ends the ability as an action, or is reduced to 0 hitpoints, all affected creatures return to an unoccupied space within 30 ft of their positions before being teleported into the demiplane.

Spellcasting (Psionics). The dragon has a number of spells prepared each day, that it can cast without using any spell components. It uses Intelligence as the spellcasting ability (Spell Save DC 25, +16 to hit with spell attacks.)

• At will: telekinesis, see invisibility, detect thoughts, gift of gab, mind sliver (11th lvl), arcane eye, sending
• 5/day each: sleep, dissonant whispers (4th lvl), mind spike (5th lvl), hunger of hadar, black tentacles, erupting earth, bigby's hand
• 3/day each: wall of fire, incendiary cloud, hold person (3rd lvl), bones of the earth, ashardalon's stride (5th lvl), raulothim's lance, plane shift, flame strike, destructive wave, synaptic static
• 1/day: psychic scream, earthquake, firestorm, illusory dragon, weird
  

Change Shape. The dragon magically transforms into any creature, that is Medium or Small, with a challenge rating equal or lower to its own. It retains its original statistics (except for size). The transformation ends when the dragon dismisses it as a bonus action, or is reduced to 0 hitpoints.

Psychic Step. The dragon magically teleports to an unoccupied space within 45 feet of it.

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.

Detect. The dragon makes a Wisdom (Perception) check.

Tail Attack. The dragon makes a tail attack.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 25 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

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

Pyroclasm (Costs 3 Actions). The dragon triggers a volcanic explosion at a point it can see within 60 feet of it. The earth erupts, sending molten rocks in every direction. Every creature within a 30 ft radius, centered on the chosen point, must succeed on a DC 25 Dexterity saving throw, or take 11 (2d10) fire damage and 11 (2d10) bludgeoning damage and be knocked prone. The affected area becomes difficult terrain until the start of the dragon's next turn.