Adult Regorium Dragon

Regorium Elemental. The Regorium Adult Dragon is a Regorium creature and thus any creature hit by it's attacks must make the requisite saving throws.

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

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: +13; to hit, reach 10 ft., one target. Hit: 20 (2d10 + 10); piercing damage plus 4 (1d8); necrotic damage.

Claw. Melee Weapon Attack: +13; to hit, reach 5 ft., one target. Hit: 18 (3d6 + 6); slashing damage.

Tail. Melee Weapon Attack: +13; to hit, reach 15 ft., one target. Hit: 16 (2d8 + 6); bludgeoning damage.

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 14 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.

Regorium Breath (Recharge 5–6); The dragon exhales a crystalline blast in a 60-foot cone. Each creature in that area must make a DC 19 Constitution saving throw, taking 54 (12d8); Regorium Corruption damage on a failed save, or half as much damage on a successful one and forcing creatures to make Regorium corruption saving throws (CON 18).

Explosive gem (Recharge 2-3). The ancient amethyst dragon spits a crystalline chunk  up to 75ft and it explodes in a 20ft radius where it lands. Each creature within the radius must make a DC 18 Dexterity saving throw or take 39 (13d6) bludgeoning damage and make Regorium Constitution saving throws (DC 18)

Crystal Shatter. When the Dragon dies, all creatures within a 40 foot sphere centered on it must make a DC 16 dexterity saving throw. A creature takes 25 (3d10+10) piercing damage and must deal with Regorium crystal corruption (DC 18 Con Save) on a failed save.

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 10 feet of the dragon must succeed on a DC 19 Dexterity saving throw or take 13 (2d6 + 6); bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.