Ancient 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, (DC 18 CON).

Terrain Walk. The dragon can move across and climb surfaces without needing to make an ability check. Additionally, difficult terrain doesn't cost it extra movement.

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: +14; to hit, reach 15 ft., one target. Hit: 19 (2d10 + 8); piercing damage plus 9 (2d8) elemental damage.

Claw. Melee Weapon Attack: +16; to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8); slashing damage.

Tail. Melee Weapon Attack: +10; to hit, reach 20 ft., one target. Hit: 17 (2d8 + 8) 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 16 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 3–4); The dragon exhales a blast of Regorium crystals in a 90-foot cone. Each creature in that area must make a DC 22 Constitution saving throw, taking 72 (16d8); regorium damage on a failed save, or half as much damage on a successful one, all creatures that are hit must make Regorium saving throws.

Summoning Roar (Recharge 6-8); The Dragon Roars while digging it's claws into the ground, infusing the surrounding area with Regorium Crystals. By doing so it will corrupt all nearby creatures that fail the saving throws, (if it is a party member they will fight for the dragon for one turn) and will summon 1d12 other Regorium creatures to it's aid.

Crystal Shatter. When the Dragon dies, all creatures within a 60 foot sphere centered on it must make a DC 18 dexterity saving throw. A creature takes 30 (4d10+10) piercing damage and must deal with Regorium crystal corruption (DC 18 Con Save) on a failed save.

Smack. If the Dragon is hit by an attack it can choose to swing it's tail at an opponent and if it hits it will knock the aggressor backwards 15 ft.

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 22 Dexterity saving throw or take 15 (2d6 + 8); bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.