Cataclysm Dragon

Limited Magic Absorption. The dragon’s horn absorbs incoming magic. When a spell targets the dragon, or the dragon is included in its area, it is treated as if cast at one level lower. If this reduces the spell below the level required to function, the spell has no effect and is absorbed.

For each level of a spell absorbed this way, the dragon gains 1d10 temporary hit points. Cantrips and 1st-level spells automatically fail to affect the dragon and grant it 1d10 temporary hit points. For example, a 2nd-level spell must be cast using a 3rd-level slot to affect the dragon.

The dragon also has advantage on saving throws against spells and other magical effects.

Immutable Form. The dragon is immune to any spell or effect that would alter its form.

Unusual Nature. The dragon doesn’t require air, food, drink, or sleep.

Legendary Resistance (4/Day). 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 Spellcasting to cast Shatter (level 3 version) or two to cast a 1/day spell.

Rend. Melee Attack Roll: +16, reach 15 ft. Hit: 18 (2d8 + 9) Slashing damage plus 11 (2d10) Lightning damage. The dragon may choose to grapple and restrain the target (escape DC 23).

Cataclysm Breath (Recharge 5–6). Dexterity Saving Throw: DC 23, each creature in a 120-foot-long, 20-foot-wide Line. Failure: 44 (8d10) Lightning damage plus 44 (8d10) Necrotic damage. Success: Half damage. A creature dies if the breath reduces it to 0 hit points. Additionally, any Medium or smaller Humanoid, Dragon or Beast killed by the breath’s damage, as well as every corpse of such a creature within the cone, becomes a zombie under the dragon’s control. The zombie acts on the dragon’s initiative but immediately after the dragon’s turn. Absent any other command, the zombie tries to kill any non-Undead creature it encounters.

Spellcasting. The dragon casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 22):

At Will: Feather Fall, Invisibility, Mage Hand, Shatter (level 3 version)
1/Day Each: Scrying, Sending, Soul Cage, Circle of Death, Chain Lightning, Lightning Storm

Cloaked Flight. The dragon uses Spellcasting to cast Invisibility on itself, and it can fly up to half its Fly Speed.

Discharge. The dragon expends 17 (3d10) temporary hit points and deals damage in a 15 ft. radius out from itself. Creatures in this area must make a DC 22 Dexterity Saving throw or take 17 (3d10) Lightning or Necrotic damage and be Paralyzed for 1 minute. Creatures take half damage on a success.