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Core-Swallower

The Glacial Vanguard is rumored to have catalogued and tracked a specimen under the designation “The Pale Ledger.” No reliable account explains how the creature reached the vents or why its feeding surges coincide with disturbances beneath the basin. Its heart remains luminous after death, a pulse without an agreed explanation.

Monsters
Gargantuan monstrosity · CR 19 · Unique

Core-Swallower

A starving, nine-headed geothermal hydra whose feeding surges destabilize the vents and deepen the unexplained pulse below the Frostmelt basin.
AC19
HP351/351
CR19
Init
STR
DEX
CON
INT
WIS
CHA
ConditionsNone

Combat

AC 19 · HP 351 · CR 19

Core-Swallower

Gargantuan monstrosity (hydra), chaotic evil
Armor Class19
Hit Points351 (19d20 + 152)
Speed
40 ft.,
Swim 60 ft.
STR
DEX
CON
INT
WIS
CHA
Saving ThrowsConstitutionWisdom
Damage Resistances
Fire,
Bludgeoning, Piercing, and Slashing damage from nonmagical attacks
Immunities
Cold
Weaknesses
Force and Radiant damage suppress Volcanic Regeneration until the end of its next turn.
It cannot use Volcanic Regeneration when more than 5 feet from magma, a Furnace heat zone, or a vent.
If reduced to 40 Hit Points or fewer and unable to reach magma, it withdraws if it can.
Senses
Darkvision 120 ft.,
Tremorsense 60 ft.,
Passive Perception 12
Languages
None
Challenge19
Initiative+2 (12)
Proficiency Bonus+6
Passive Perception12
Grapple Escape DC22
Many-Headed. The Core-Swallower has nine heads. If it takes 40 or more damage in a single turn, one head dies. It regrows two heads at the start of its next turn. While it has at least five heads, it has Advantage on saving throws against being Blinded, Deafened, Frightened, Stunned, or knocked Unconscious.
Legendary Resistance (3/Day). If the Core-Swallower fails a saving throw, it can choose to succeed instead. It can use this trait three times per day.
Volcanic Regeneration. At the start of its turn, the Core-Swallower regains 15 Hit Points if it is within 5 feet of magma, a Furnace heat zone, or a vent. This regeneration stops until the end of its next turn if it takes Radiant or Force damage.

Actions

Multiattack. The Core-Swallower makes three Bite attacks and two Claw attacks.
Bite. Melee Attack Roll: +14, reach 15 ft. Hit: 21 () Piercing damage plus 7 () Cold damage. If the target is Huge or smaller and the Core-Swallower has fewer than three creatures grappled, the target has the Grappled condition (escape ).
Claw. Melee Attack Roll: +14, reach 15 ft. Hit: 24 () Slashing damage.
Thermal-Shock Breath. Recharge 5–6. The Core-Swallower exhales two 60-foot Cones at once, one of fire and one of freezing vapor. Each creature in either Cone makes a Dexterity saving throw. On a failure, a creature takes 31 () Fire damage in the fire Cone and 31 () Cold damage in the cold Cone. A creature in both Cones takes both damage types. On a success, it takes half damage only.
Magma Surge. Recharge 6. Magma erupts around the Core-Swallower. Each creature within 5 feet makes a Dexterity saving throw, taking 27 () Fire damage on a failure or half as much on a success.

Reactions

Shifting Carapace. When the Core-Swallower takes Cold or Fire damage, it gains Resistance to that damage type until the start of its next turn.

Legendary Actions

Undertow. Each creature within 30 feet of the Core-Swallower that is in water makes a Strength saving throw. On a failure, the creature is pulled up to 15 feet toward the Core-Swallower.
Snap. The Core-Swallower makes one Bite attack.
Rime Pulse (costs 2 actions). Each creature within 20 feet makes a Constitution saving throw. On a failure, it takes 18 () Cold damage and its Speed becomes 0 until the end of its next turn. On a success, it takes half as much damage only.
Role
Solo brute and area controller
Built for
Level 17–20; calibrate action economy and terrain for party size

Notes for the game master

Custom D&D 5E monster using the supplied statistics and features.
For Many-Headed, a head dies when the Core-Swallower takes 40 or more damage during a single turn. At the start of its next turn, it regrows two heads. This does not change its listed attacks.
Volcanic Regeneration restores 15 Hit Points at the start of its turn only while it is within 5 feet of magma, a Furnace heat zone, or a vent. Radiant or Force damage suppresses this regeneration until the end of its next turn.
The Core-Swallower can grapple no more than three creatures at once with its Bite attacks.

Tactics

  1. Opening

    It erupts from the water or a vent channel, sweeping the space with Thermal-Shock Breath before closing on the nearest cluster of creatures.

  2. Mid-fight

    Use Multiattack against nearby prey, Grapple up to three creatures, and exploit Thermal-Shock Breath when its two cones can catch separate groups or overlap on a dangerous target. Use Magma Surge when several foes crowd its reach.

  3. End of the fight

    When reduced to 40 Hit Points or fewer and unable to reach magma, the Core-Swallower withdraws if it can. If trapped, it uses its remaining heads to seize prey and hold ground near heat.

  4. Retreat

    This is a starving animal, not a strategist. Lure it toward a safer heat source or away from a failing conduit. It retreats at 40 Hit Points or fewer if it cannot reach magma.

How it fights

Use water and narrow channels to make Undertow dangerous, but leave routes for characters to reposition.
Use the two cones of Thermal-Shock Breath to pressure separated groups, or overlap them on a key target.
Use Bite attacks to grapple characters who threaten its access to heat, then attack other nearby foes with its remaining bites and claws.
Stay within reach of a vent when possible to benefit from Volcanic Regeneration, but do not retreat into an inaccessible position that removes counterplay.

Who it attacks first

Creatures blocking its route to a vent or magma.
Nearby prey it can grapple and hold.
Groups positioned to threaten its access to heat.

Works well with

A flooded geothermal cavern lets Undertow pull characters toward the Core-Swallower while its breath attacks threaten multiple lanes.
A failing conduit creates a moving objective, requiring the party to protect workers or redirect the creature while fighting.
Nearby vents provide clear, tactical choices: contest the heat source, force the creature away, or use it as bait.

How to beat it

Force or Radiant damage suppresses Volcanic Regeneration until the end of its next turn.
Draw it away from magma, a Furnace heat zone, or a vent to deny its regeneration.
Spread out to reduce the number of creatures caught in Thermal-Shock Breath and Magma Surge.
Escape its grapples promptly, and avoid clustering within reach of its claws.

Ways to avoid a fight

Lead it away from a failing conduit toward an abandoned or safer vent. A clear heat trail and a route free of obstructions can avert a fight.
Use coordinated signals and controlled heat releases to redirect its attention, while keeping rescuers clear of its reach.
Calm or redirect the animal rather than kill it, preserving the vent network and avoiding further harm to the local heat supply.

Ecology

A gargantuan, nine-necked mass of black volcanic plates and salt-glazed hide hauls itself through the water. Its mouths steam in alternating clouds, one exhaling furnace heat, the next a dry, needling cold. Orange light pulses through seams in its chest like a coal seen beneath cracked ice; its claws scrape stone with a sound like a ship dragged over gravel.

Behavior

The Core-Swallower follows heat and the vibration of moving prey. It attacks anything that blocks its path to a vent, swallowing trapped creatures when it can. It does not guard territory out of malice, but it reacts violently to interruption while feeding.

How it lives

A rare, ravenous predator drawn to the geothermal arteries beneath the Frostmelt Archipelago. Its body tolerates intense cold and heat, but its appetite for vent-fed warmth can turn a local disturbance into a settlement-wide crisis.

Lair

It shelters in submerged volcanic chambers beside active vents. Its lair is littered with fractured conduit stone, scorched anchors, and the remains of creatures drawn too close to the warmth.

Habitat
Geothermal basins, flooded volcanic conduits, vent fields, and magma-adjacent caverns in the Frostmelt Archipelago.

Signs that it is near

A wet, mineral smell mixed with scorched stone and sudden frost.
Deep claw furrows crossing flooded rock, their edges glazed in ice.
A steady orange glow beneath the water, pulsing through cracks in the seabed.
Vent output surging and faltering in time with distant impacts.

Tags

monster,
hydra,
geothermal,
Frostmelt Archipelago,
Tier 4

Lore

The Glacial Vanguard is rumored to have catalogued and tracked a specimen under the designation “The Pale Ledger.” No reliable account explains how the creature reached the vents or why its feeding surges coincide with disturbances beneath the basin. Its heart remains luminous after death, a pulse without an agreed explanation.

What drives it

Reach a source of geothermal warmth and feed. It seeks survival, not conquest, and may be diverted if offered a safer route to heat.

After the fight

When the Core-Swallower dies, its heart-glow persists beneath the cooling hide, beating against the silence. The nearby vent may recover, but each feeding surge has worsened the disturbance beneath the basin, and something may still be listening.

Variants

Brine-Sour Form: Its bites carry poison. Add poison damage to Bite attacks as appropriate for the party's tier.
Vent-Sated Form: It is slower and calmer, and its Volcanic Regeneration restores half as many Hit Points.
The Pale Ledger: A rumored specimen catalogued and tracked by the Glacial Vanguard.

Hooks

Plot hooks

Every feeding surge worsens a disturbance beneath the basin. The party must decide whether to kill the Core-Swallower or divert it before the next surge.
The creature's heart continues glowing after death, and its rhythm appears to echo a pulse from below.
A Glacial Vanguard record identifies the beast as “The Pale Ledger,” but the remaining entries describe tracking data without revealing what the organization hoped to learn.

How to use it

Use it as a high-stakes Tier 4 encounter near a threatened vent, where defeating it and protecting the heat supply are competing demands.
Make the battlefield legible with flooded channels, narrow ledges, vent mouths, and visible heat zones that affect both regeneration and movement.
Present signs of its hunger first, giving characters a chance to divert it rather than fight.
Track its heads visibly so the party can understand the head-loss and regrowth cycle.

Treasure

A heart-core that remains faintly luminous after death, valuable as evidence or a dangerous research specimen.
Glacial Vanguard tracking records associated with the rumored designation “The Pale Ledger.”
Salvage from broken conduit machinery caught beneath the beast's lair.

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