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Monolith Sentinel - AI-generated fantasy Monster

Monolith Sentinel

The Monolith Sentinel is an engineered weapon rather than a living guardian. Siphon-Lichs use these constructs to protect heat-extraction works, and their cores can preserve traces of the network they served. This Sentinel's core contains an incomplete map of siphon lines beneath the basin. A command crystal left by its builders points toward Monolith Omega.

Monsters
Gargantuan construct · CR 20 , 25,000 XP · Unique

Monolith Sentinel

A mobile siphon engine built to guard extraction sites, drain heat from living bodies, and defend routes leading toward Monolith Omega.
AC19
HP385/385
CR20 , 25,000 XP
Init
STR
DEX
CON
INT
WIS
CHA
ConditionsNone

Combat

AC 19 · HP 385 · CR 20 , 25,000 XP

Monolith Sentinel

Gargantuan construct (construct), neutral evil
Armor Class19
Hit Points385 (22d20 + 154)
Speed
40 ft.,
Swim 40 ft.
STR
DEX
CON
INT
WIS
CHA
Saving ThrowsConstitutionWisdom
Damage Resistances
Cold damage,
Bludgeoning damage from nonmagical attacks,
Piercing damage from nonmagical attacks,
Slashing damage from nonmagical attacks
Immunities
Poison damage,
Psychic damage,
Charmed,
Exhaustion,
Frightened,
Paralyzed,
Petrified,
Poisoned
Weaknesses
Disabling nearby siphon pylons reduces the Sentinel's legendary actions for the round.
A valid command crystal can redirect it from a defended vent.
Its low intelligence makes it vulnerable to lures, misdirection, and carefully staged sabotage.
Senses
Blindsight 120 ft.,
Passive Perception 11
Languages
Understands Common and Giant but cannot speak
Challenge20 , 25,000 XP
Initiative+0 (10)
Proficiency Bonus+6
Aura of Absolute Zero30-foot radius; 22 (4d10) Cold damage at the start of the Sentinel's turn, not reduced by Cold Resistance
Legendary Resistance (3/Day). If the Sentinel fails a saving throw, it can choose to succeed instead. It can do so three times per day.
Aura of Absolute Zero. At the start of each of the Sentinel's turns, each creature within 30 feet takes 22 () Cold damage. Resistance to Cold does not reduce this damage.

Actions

Multiattack. The Sentinel makes three Adamantine Limb attacks, or two Adamantine Limb attacks and one Cryo-Lance attack. It can make only one Cryo-Lance attack.
Adamantine Limb. Melee Attack Roll: +16, reach 20 ft. Hit: 28 () Bludgeoning damage plus 13 () Cold damage.
Cryo-Lance. Ranged Attack Roll: +16, range 120/360 ft. Hit: 31 () Piercing damage plus 18 () Cold damage.
Thermal Extraction Beam. Recharge 5–6. Constitution Saving Throw: , each creature in a 120-foot Line that is 10 feet wide. Failure: 55 () Cold damage plus 33 () Force damage, and the target loses one unexpended spell slot of its highest level, if it has one. A Thermalist instead loses 2 Heat Points. Success: Half damage only, with no resource loss. The beam does not recharge from a slot or Heat Point lost.
Core Vent. Recharge 4–6. Constitution Saving Throw: , each creature in a 60-foot Cone. Failure: 45 () Fire damage. Success: Half damage.

Reactions

Cryo-Plate Deflection. When the Sentinel is hit by a ranged attack, it gains a bonus to AC against that attack and any other ranged attacks until the start of its next turn.

Legendary Actions

Trench Traverse. The Sentinel moves up to half its Speed without provoking Opportunity Attacks.
Plate Shove. One creature the Sentinel can see within 20 feet makes a Strength saving throw. On a failure, it is pushed up to 20 feet away and has the Prone condition.
Crystal Discharge (costs 2 actions). Each creature within 15 feet makes a Dexterity saving throw, taking 18 () Force damage on a failure or half as much damage on a success.
Cold-Sink Pulse (costs 2 actions). If Thermal Extraction Beam is not recharged, the Sentinel releases a 30-foot Cone. Each creature in the area makes a Constitution saving throw, taking half the beam's damage on a failure or half as much damage on a success. A target that fails also loses one unexpended spell slot of its highest level, if it has one, or 2 Heat Points if it is a Thermalist. This use does not recharge the beam.
Role
Solo boss and heat-siphon guardian
Built for
20

Gear

  • Command Crystal (Control component, Unique)An embedded shard keyed to the Sentinel's command protocol. A valid crystal can redirect it from a defended vent.
  • Siphon Map Core (Construct component, Recoverable after the Sentinel is disabled)A hardened matrix of siphon conduits and engraved route marks inside the core. It holds an incomplete map of siphon lines beneath the basin.

Augmentations

Adamantine frame and articulated siege limbs
Cryogenic crystal lattice
Thermal siphon core
Pressure-sealed joints

Notes for the game master

Custom D&D 5E monster using the supplied statistics and features.
Legendary Resistance (3/Day): If the Sentinel fails a saving throw, it can choose to succeed instead.
Aura of Absolute Zero deals 22 (4d10) Cold damage to each creature within 30 feet at the start of the Sentinel's turns. Cold Resistance does not reduce this damage.
The aura's exception for Cold Resistance is a custom feature and does not otherwise alter Cold Resistance.
The listed challenge rating and XP are provided as supplied.

Tactics

  1. Opening

    It identifies intruders by heat and movement, then opens with Thermal Extraction Beam if several targets line up. Otherwise, it closes toward the warmest creature and begins its aura damage.

  2. Mid-fight

    It advances through difficult terrain without hesitation, striking nearby threats while firing Cryo-Lance at distant targets. It uses Thermal Extraction Beam against a line of clustered foes or anyone relying on potent magic.

  3. End of the fight

    When badly damaged or its pylons are disabled, it keeps its core facing the largest cluster of intruders, using Core Vent to punish close pursuit and Cold-Sink Pulse to drive enemies away from its position.

  4. Retreat

    The Sentinel does not flee, bargain, or feel fear. It continues its assigned defense until destroyed, disabled by sabotage, or redirected by a valid command crystal.

How it fights

Use its 120-foot blindsight to track creatures through murky water, darkness, and obscuring vapor.
Maintain pressure with the aura while using Trench Traverse to reposition around cover.
Aim Thermal Extraction Beam through multiple targets, but avoid repeating the same line if the party has adapted.
Use Plate Shove to break formation or push a target away from a pylon.
Make the pylons visible and interactable so characters can choose sabotage over direct damage.

Who it attacks first

The warmest nearby creature or the largest concentrated heat source.
Creatures attacking siphon pylons or attempting to reach its command mechanism.
A spellcaster or Thermalist positioned in the path of Thermal Extraction Beam.
Intruders attempting to cross the defended vent or extraction route.

Works well with

Siphon pylons provide the Sentinel with a natural battlefield objective and reinforce its heat-draining theme.
A siphon crew or lesser construct can keep intruders occupied while the Sentinel controls the main approach.
The Sentinel's Cold-Sink Pulse can scatter clustered foes into areas threatened by vent terrain.

How to beat it

Disable the siphon pylons around it. Each pylon shut off removes one of its legendary actions for a round.
Use cover, spacing, and broken sight lines to reduce the number of creatures caught in its line and cone attacks.
Force it to spend movement navigating terrain, then attack from multiple directions.
Force damage and adamantine-breaking tools can harm it; deliberate sabotage of its support machinery can weaken its combat routine.

Ways to avoid a fight

Present a valid command crystal to redirect the Sentinel from its defended vent.
Disable its siphon pylons to reduce its legendary-action economy and interrupt the station's heat draw.
Recover its embedded route map or inspect its core for evidence of who built or deployed it.

Ecology

A walking slab of dark adamantine and milky crystal, the Sentinel rises like a piece of the seabed torn upright. Frost crawls across its broad plates in branching veins. Its head is an unlit, featureless block, while a blue-white seam in its chest pulses with the measured throb of a buried bell. Each step sends grit skittering across the floor and a pressure wave through the water.

Behavior

It advances with the patience of a tide and the certainty of a machine following a single instruction. The core brightens when it senses warmth, while its plates click and groan as they reposition. It ignores pain, speech, and pleas, but recognizes a valid command crystal and can be redirected from its post.

How it lives

These engines do not reproduce or feed. Siphon-Lichs deploy them as mobile heat sinks wherever a vent, route, or extraction site must be held. The Sentinel's stolen warmth is routed into siphon infrastructure, leaving water and stone brittle with cold.

Lair

Its assigned post is usually a broad platform of pressure-sealed stone surrounded by siphon pylons. Vent mist rolls over the floor, while the pylons hum at different pitches as they draw warmth from the surrounding water.

Habitat
Abyssal Trench, Sovereign Deep, pressure-sealed siphon works, and cold extraction sites around geothermal vents.

Signs that it is near

A spreading skin of frost on stone and metal, even beside a warm vent.
A low, repeating pulse felt through boots, hull plates, or the bones of the jaw.
Freshly drained heat leaving a sharp mineral smell and a cloud of brittle ice crystals.
Deep gouges in the floor, each spaced as if made by a deliberate, heavy stride.

Tags

construct,
boss,
siphon engine,
cold,
abyssal trench,
campaign canon

Lore

The Monolith Sentinel is an engineered weapon rather than a living guardian. Siphon-Lichs use these constructs to protect heat-extraction works, and their cores can preserve traces of the network they served. This Sentinel's core contains an incomplete map of siphon lines beneath the basin. A command crystal left by its builders points toward Monolith Omega.

What drives it

Enforce its standing command to protect siphon infrastructure and drain heat from intruders.

After the fight

When its core goes dark, pressure releases in a low, shuddering groan. Frost sloughs from the plates, exposing siphon conduits and a crystal map fragment that may still point toward Monolith Omega.

“Siphon-Lichs use these constructs to protect heat-extraction works, and their cores can preserve traces of the network they served.”
From its lore

Variants

The Ballad's Bell: Huge, 300 hit points, and no Cold-Sink Pulse.
The Cold Tithe: A crystal snare targets one creature within 60 feet, which must succeed on a DC 21 Strength saving throw or have the Restrained condition. The target can repeat the save at the end of each of its turns, ending the condition on a success.
The Quiet Writ: Its aura extends 15 feet and creates an area of silence. It has one additional reaction each round.

Hooks

Plot hooks

The Sentinel's core map identifies a damaged siphon route that leads toward Monolith Omega.
A command crystal recovered from a wreck activates the construct, but its instructions are incomplete.
The party finds a crew trapped near the defended vent, with the Sentinel drawing heat from the only safe route out.

How to use it

Use as a CR 20 set-piece guardian at a siphon station, vent complex, or deep-sea approach to Monolith Omega.
Place siphon pylons in exposed but hazardous positions so disabling them becomes a tactical objective amid the fight.
Give the party a chance to discover the command crystal or pylon controls before the Sentinel activates.
Its aura and resource-draining beam make it especially dangerous in a prolonged fight. Use clear telegraphing and available cover to keep the encounter playable.

Treasure

The intact command crystal, usable to redirect the construct if its protocol accepts the bearer.
An incomplete map of siphon lines beneath the basin, preserved within the core.
Adamantine plates and cryogenic crystals that can be salvaged with time and suitable tools.

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