Adult Electrum Dragon

Magical Conduction. If the dragon is hit by a spell attack, all creatures within 15 feet of the dragon take half of the damage dealt by the attack. 

Draconic Focus. The dragon may concentrate on up to 3 different spell instances at once. When subjected to a concentration saving throw, the dragon only rolls its saving throw once, breaking concentration on all spells on a failure. 

Legendary Resistance (3/Day, or 4/Day in Lair). If the dragon fails a saving throw, it can choose to succeed instead.

Multiattack. The dragon makes three Rend attacks. It can replace one attack with a use of (A) Blinding Maw or (B) Spellcasting to cast Witch Bolt.

Rend. Melee Attack Roll: +10, reach 10 ft. Hit: 14 (2d8 + 5) Slashing damage plus 5 (2d4) Fire or Cold damage, chosen by the dragon. 

Tail. Melee Attack Roll: +10, reach 25 ft. Hit: 8 (1d6 + 5) Bludgeoning damage and the target is knocked Prone

Glass Breath (Recharge 5–6). The dragon breathes a surge of molten glass followed by a wave of cooling air, solidifying the molten glass immediately. Dexterity Saving Throw: DC 19. Each creature in a 60 foot Cone. Failure: 26 (4d12) Fire damage plus  26 (4d12) Cold damage and the target is Restrained by the solidifying glass. Failure by 10 or more: In addition to the typical failure effects, the creature is knocked Prone and is considered to be suffocating until freed from being restrained. Success: Half damage only. A creature Restrained in this way may use its action to attempt a DC-19 Strength saving throw, ending the condition on itself on a success. A creature within 5 feet of the Restrained creature may use its action to attempt a DC-17 Strength saving throw, ending the Restrained condition on the creature on a success. 

Blinding Maw. The dragon opens its mouth, revealing a blindingly bright light. Dexterity Saving Throw: DC 19, each creature in a 60-foot Cone that can see the dragon. Failure: The target is Blinded for 1d4 rounds. A blinded creature may repeat the saving throw at the end of each of their turns, ending the effect on themselves on a success. After this attack, the affected area shines with Bright Light in a 120 foot cone and Dim Light for an additional 120 feet. 

Spellcasting. The dragon casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 20, +12 to hit with spell attacks):

At Will: Detect Magic, Witch Bolt, Shapechange (Beast or Humanoid form only, no Temporary Hit Points gained from the spell, and no Concentration or Temporary Hit Points required to maintain the spell), Detect Thoughts, Thaumaturgy,
3/Day Each: Searing Smite (level 2 version, dealing Cold or Fire damage, chosen by the dragon upon casting the spell)
2/Day Each: Misty Step

Frostburn Smite (3/Day). The dragon casts Searing Smite (level 2 version, dealing Cold or Fire damage, chosen by the dragon upon casting the spell). 

Blink (2/Day). The dragon uses its Spellcasting to cast Misty Step

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature’s turn, the dragon can expend a use to take one of the following actions. The dragon regains all expended uses at the start of each of its turns.

Sapping Bolt. The dragon uses Spellcasting to cast Witch Bolt (level 2 version).

Tail Whip. The dragon makes one Tail attack.

Bolt Snap. Dexterity Saving Throw: DC 17, one creature affected by Witch Bolt by the dragon. Failure: 19 (3d12) Lightning damage. Success: No damage. Success or Failure: The dragon ceases concentration on the instance of Witch Bolt targeting the creature which it targets this legendary action on.