Belephaion

Lightning Scales.
When Belephaion takes damage from a metal melee weapon, the attacker takes 1d8 lightning damage.

Rebuke the Unworthy.
When Belephaion reaches 100 HP, each enemy in a 20-foot radius centered on Belephaion must make a DC 17 Wisdom saving throw, taking 21 (6d6) radiant damage and losing their reaction on a failed save. Half damage on a success.

Innate Spellcasting (1/Day).
Belephaion can innately cast the following spells, requiring no material components. His spellcasting ability is Wisdom (save DC 17):

• Ascendant Storm (reflavored Fire Storm) 1/Day

Multiattack. The dragon makes three Rend attacks.

Rend. Melee Attack Roll: +9, reach 10 ft. Hit: 12 (2d6 + 5) Slashing damage plus 5 (1d10) Lightning damage.

Chain Lightning Breath (Recharge 5–6).
Belephaion exhales radiant-charged lightning that arcs between targets. One target in range (90 ft) makes a DC 19 Dex save, taking 55 (10d10) lightning damage on a fail, or half on success. The lightning then arcs to additional creatures within 10 ft of the previous target(s), who must each also make a save.

Priest of Takhisis (3/Day).
As a bonus action, Belephaion may cast one of the following, using Wisdom as his spellcasting stat: Bane, Dispel Magic, Healing Word, or Blindness/Deafness.

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

Legendary Actions (2/Round):

Crackling Step. (1 Action)
Belephaion shifts up to 10 feet without provoking opportunity attacks.

Tail Swipe. (1 Actions)
The dragon makes one Rend attack.

Sonic Boom. (1 Actions)
The dragon uses Spellcasting to cast Shatter. The dragon can't take this action again until the start of its next turn.