A mithral dragon can spit a line of metallic shards. Targets in its path take damage that varies upon the Dragons age:

Young - 50 ft long, 5 ft wide line of metallic shards. Targets in its path take 21 (6d6) magical slashing damage and lose another 5 hit points from bleeding at the start of their turns for 6 rounds; slashing damage is halved by a successful DC 12 Dexterity saving throw, but bleeding damage is not affected. Only magical healing stops the bleeding before 6 rounds. The shards dissolve into wisps of smoke 1 round after the breath weapon’s use.

Adult -  60-foot-long, 5-foot-wide line of metallic shards. Targets in its path take 42 (12d6) magical slashing damage and lose another 8 hit points from bleeding at the start of their turns for 6 rounds; slashing and bleed damage are halved by a successful DC 18 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.

Ancient - 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. 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 traveled. Otherwise they dissolve into wisps of smoke 1 round after breath weapon is used.

Damage Resistances: Bludgeoning, Piercing, and slashing from non-magical weapons.

Damage Immunities: Acid and Thunder        Conditional Immunities:  Charmed

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

Armor Class:  Natural Armor -   Young  16      Adult  18        Ancient  20

Hit Points:    Young   92 (16d8 + 20)    Adult   184 (16d12 +80)   Ancient   297  (17d20 +119)

Speed:           Young      50ft, Fly 60ft     Adult  and Ancient:  40 ft  Fly 80ft 

Abilities:

Young:    STR  13 (+1)   DEX 22 (+6)  CON  13 (+1)  INT  14 (+2)  WIS  15 (+2)  CHA 14 (+2)  

Adult:      STR  27 (+8)   DEX 18 (+4)  CON  21 (+5)  INT  20 (+5)  WIS  21 (+5)  CHA 20 (+5)  

Ancient:  STR  29 (+9)   DEX 16 (+3)  CON  25 (+7)  INT  24 (+7)  WIS  25 (+7)  CHA 24 (+7)   

Savings Throws:

Young:     Dex +9  Con +4   Wis +5    Cha +5

Adult:      Dex +9  Con +10  Wis +10 Cha +10

Ancient:  Dex +9  Con +13  Int  +13  Wis +13  Cha +13

Senses:

Blindsight 60ft, Darkvision 120ft, Passive Perception:  Young 15,  Adult 20,  Ancient 23

Languages: Natural  Celestial, Draconic, Primordial and Common

C/R Rating:   Young - 6 (2300 xp)  Adult - 14 (11500 xp)  Ancient 18 (20000 xp)

The dragon’s innate spellcasting ability is Intelligence:

It can cast the following spells w/o any material components.

At Will:  Tongues

Young:   3/Day - Enhance Ability,   Adult:  5/Day Dispel Magic, Enhance Ability   Ancient: 5/Day Counterspell, Dispel Magic, Enhance Ability

Adult:  The dragon is a 6th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 18, +10 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, 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

Ancient: 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

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.

Young:

Multiattack. The dragon makes one bite attack and two claw attacks.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) piercing damage.

Claw. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 14 (2d10 + 3) slashing damage, and the target loses 3 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 3 hp per round for each bleeding wound it has taken from a mithral dragon’s claws.

Breath Weapon (Recharge 5-6). A mithral dragon can spit a 50-foot-long, 5-foot-wide line of metallic shards. Targets in its path take 21 (6d6) magical slashing damage and lose another 5 hit points from bleeding at the start of their turns for 6 rounds; slashing damage is halved by a successful DC 12 Dexterity saving throw, but bleeding damage is not affected. Only magical healing stops the bleeding before 6 rounds. The shards dissolve into wisps of smoke 1 round after the breath weapon’s use.

Adult:

 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: 19 (2d10 + 8) piercing damage.

Claw. Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 15 (2d6 +8) slashing damage, and the target loses 4 hit points 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 4 hp per round for each bleeding wound it’s taken from a mithral dragon’s claws.

Tail. Melee Weapon Attack: +13 to hit, reach 15 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 18 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 42 (12d6) magical slashing damage and lose another 8 hit points from bleeding at the start of their turns for 6 rounds; slashing and bleed damage are halved by a successful DC 18 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 21 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.

Ancient: 

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.