Ancient Mithral Dragon

• Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.
• Innate Spellcasting. the dragon's innate spellcasting ability is Intelligence (spell save DC 21). It can innately cast the following spells, requiring no material components:
  
  at will: tongues
  
  5/day each: counterspell, dispel magic, enhance ability
• Mithral Shards. Ancient mithral dragons can choose to retain the mithral shards of their breath weapon as a hazardous zone of spikes. Treat as a spike growth zone that does 2d8 magical slashing damage for every 5 feet travelled.
• Spellcasting. the dragon is a 15th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 21, +13 to hit with spell attacks). It requires no material components to cast its spells. The dragon has the following wizard spells prepared:
  
  cantrips (at will): acid splash, light, mage hand, minor illusion, prestidigitation
  
  1st level (4 slots): charm person, expeditious retreat, magic missile, unseen servant
  
  2nd level (3 slots): blur, hold person, see invisibility
  
  3rd level (3 slots): haste, lightning bolt, protection from energy
  
  4th level (3 slots): dimension door, stoneskin, wall of fire
  
  5th level (2 slots): polymorph, teleportation circle
  
  6th level (1 slot): guards and wards
  
  7th level (1 slot): forcecage
  
  8th level (1 slot): antimagic field

Actions

• 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: +15 to hit, reach 15 ft., one target. Hit: 22 (2d12 + 9) piercing damage.
• Claw. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 18 (2d8 + 9) slashing damage, and the target loses 5 hit point from bleeding at the start of each of its turns for six rounds unless it receives magical healing. Bleeding damage is cumulative; the target loses 5 hp per round for each bleeding wound it's taken from a mithral dragon's claws.
• Tail. Melee Weapon Attack: +15 to hit, reach 20 ft., one target. Hit: 20 (2d10 + 9) 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 21 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of its turn, 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.
• Breath Weapon (Recharge 5-6). A mithral dragon can spit a 60-foot-long, 5-foot-wide line of metallic shards. Targets in its path take 59 (17d6) magical slashing damage and lose another 10 hit points from bleeding at the start of their turns for 6 rounds; slashing and bleed damage is halved by a successful DC 21 Dexterity saving throw. Only magical healing stops the bleeding before 6 rounds. The shards dissolve into wisps of smoke 1 round after the breath weapon's use.

Legendary Actions

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 23 Dexterity saving throw or take 18 (2d8 + 9) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.