Living Cataclysm
Traits
Legendary Resistance (3/Day).
Siege Monster. Double damage to objects.
Cataclysmic Aura. An enemy that ends its turn within 20 feet of the Living Cataclysm makes a DC 21 Dexterity save. Failure: 21 (6d6) Fire damage and is thrown 10 feet in a random direction.
Unstoppable. Nothing affects the Living Cataclysm's movement (Difficult Terrain, spells, walls — it tears through all of it, including Wall of Force).
Actions
Multiattack. Makes two Cataclysm Slam attacks and uses one Eruption.
Cataclysm Slam. Melee Attack Roll: +15, reach 20 ft. Hit: 36 (5d10 + 9) Bludgeoning damage plus 14 (4d6) Fire damage. The target makes a DC 23 Strength save. Failure: Thrown 30 feet and Prone.
Eruption. A volcanic blast at a point it can see within 60 feet. Every creature in a 20-foot-radius Sphere makes a DC 21 Dexterity save. Failure: 42 (12d6) Fire damage and Prone. Success: Half damage.
Cataclysm (Recharge 6). The Living Cataclysm erupts. Every enemy in a 60-foot Emanation makes a DC 21 Constitution save.
- Failure: 77 (14d10) Fire damage, and thrown 30 feet.
- Success: Half damage.
Legendary Actions (3/Round)
Slam. Makes one Cataclysm Slam.
Molten Step (Costs 2). Moves its Speed; the squares it crosses burn for 1 minute (a creature that ends its turn there takes 10 (3d6) Fire).
How to Use
A walking catastrophe. Tier 4's pure destruction.
Unstoppable — no obstacle works. Not even Wall of Force. The party can't slow it down.
Key
Cold damage (Resistance, not Immunity) and a straight damage race. This fight is a DPS race — the party must bring it from 350 HP, and it's doing the same to them. No running away.