Adult Obsidian Dragon

Amphibious. The dragon can breathe both air and water.

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Multiattack. The dragon makes one Bite attack and two Claw attacks.

Bite. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 18 (2d10+7) piercing damage plus 9 (2d8) force damage.

Claw. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 11 (1d8+7) slashing damage.

Singularity Breath (Recharge 5–6). The dragon creates a shining bead of gravitational force in its mouth, then releases the energy in a 60-foot cone. Each creature in that area must make a DC 20 Strength saving throw. On a failed save, the creature takes 45 (10d8) force damage, and its speed becomes 0 until the start of the dragon’s next turn. On a successful save, the creature takes half as much damage, and its speed isn’t reduced.

Spellcasting (Psionics). The dragon casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 18):

1/day each: blink, control water, dispel magic, protection from evil and good, sending

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 magically teleports to an unoccupied space it can see within 60 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.

Claw. The dragon makes one claw attack.

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

Explosive Crystal (Costs 3 Actions). The dragon spits an Obsidian Shard that that explodes at a point it can see within 60 feet of it. Each creature within a 20-foot-radius sphere centered on that point must succeed on a DC 20 Dexterity saving throw or take 13 (3d8) force damage and be knocked prone.