Cataclysm Engine

(aka the Worldbreaker, the Devourer Core, the God-Machine)
Gargantuan monstrosity (titan), unaligned

CR 22 (41,000 XP)
This is not a normal fight. This is a disaster event.

DEFENSE

Armor Class 26 (living armor plating)
Hit Points 1245 (80d20 + 574)
Speed 50 ft., burrow 30 ft. (through rubble)

Saving Throws Str +17, Con +16, Wis +11
Damage Resistances bludgeoning, piercing, slashing (non-artifact)
Damage Immunities poison, radiation*, lightning, (bludgeoning, piercing, slashing from non magical attacks)
Condition Immunities charmed, frightened, paralyzed, petrified, poisoned, stunned
Senses tremorsense 300 ft., darkvision 240 ft., passive Perception 24
Languages —
Proficiency Bonus +7

*Radiation = poison/necrotic equivalents unless you use custom rules.

CORE TRAITS

Titanic Presence

• The Cataclysm Engine cannot be knocked prone

• It ignores difficult terrain

• Creatures Huge or smaller cannot grapple it

Any creature that starts its turn within 30 ft. must succeed on a DC 20 Strength save or be knocked prone by sheer mass and shockwaves.

Citybreaker

Structures, vehicles, and terrain take double damage from the Cataclysm Engine’s attacks.

When it moves, it may move through buildings, automatically destroying non-fortified structures.

Living Reactor Core

The monster is powered by a radioactive bio-mechanical core.

• When it deals damage to a creature or structure containing power, fuel, or magic energy, it regains 20 HP

• When reduced below 50% HP, it enters Reactor Instability

Reactor Instability (Phase Change)

At 390 HP or lower, the core becomes visible beneath torn armor.

Effects:

• Attacks against it have advantage

• It loses resistance to nonmagical damage

• It gains +20 movement speed

• It makes one additional attack per Multiattack

This phase lasts until death.

OPTIONAL WORLD EFFECTS

• Cities are permanently altered

• Magic behaves unpredictably nearby

• Governments hunt it or worship it

• The monster becomes a moving plotline

ACTIONS

Multiattack

The Cataclysm Engine makes three attacks:

• One Devastating Slam

• One Claw or Bite

• One Tail Sweep or Stomp

(Phase 2: four attacks)

Devastating Slam

Melee Weapon Attack: +17 to hit, reach 25 ft.
Hit: 45 (6d10 + 10) bludgeoning damage
Target must succeed on a DC 20 Strength save or be:

• Knocked prone

• Pushed 30 ft.

• Take 10 (3d6) thunder damage from impact

Bite (Reactor Maw)

Melee Weapon Attack: +17 to hit, reach 15 ft.
Hit: 52 (6d12 + 10) piercing + necrotic damage

If the target is a construct, vehicle, or carrying a power source:

• The Cataclysm Engine regains 30 HP

Tail Sweep (Recharge 4–6)

The monster sweeps its tail in a 60-ft. arc.

Creatures in the area must make a DC 20 Dexterity save:

• Fail: 38 (6d10) bludgeoning damage and knocked prone

• Success: Half damage, not prone

Cataclysmic Stomp

Melee Weapon Attack: +17 to hit, reach 20 ft.
Hit: 40 (6d8 + 10) bludgeoning damage
Ground becomes difficult terrain, and creatures within 10 ft. take 10 damage.

Core Discharge (Recharge 5–6)

The reactor vents unstable energy.

Creatures in a 40-ft. radius must make a DC 21 Constitution save:

• Fail: 55 (10d10) radiant/necrotic damage + 1 level of exhaustion

• Success: Half damage, no exhaustion

LEGENDARY ACTIONS (3/ROUND)

Move. Moves up to half speed.
Claw Attack. Makes one claw attack.
Ruin the Field (Costs 2 Actions). Destroys terrain, collapsing cover and knocking creatures prone in a 20-ft. area.
Consume Power (Costs 3 Actions). Absorbs nearby fuel, magic, or structures; regains 50 HP.

MYTHIC ACTION

If reduced to 0 HP while Reactor Instability is active:

Meltdown Protocol (Mythic Phase)

• HP resets to 250

• Armor fully breaks off

• All attacks deal +10 damage

• Core Discharge recharges every round

• At the end of each round, the battlefield takes damage

This phase lasts 3 rounds max before total collapse.