Adamant Sentinel

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Image prompthuge construct guardian three times human height, seamless dark adamantine plate over a monolithic blocky frame, faceless helm-head with a single dim rune-glow, siege-hammer fists, standing astride a mountain pass gateway barring the way, immovable stance, faint pale force-light at its joints, hewn megalithic stone blocks, cold thin light, a guarded chamber hinted beyond, worm's-eye low angle emphasizing scale, dark metal and grey palette, dark fantasy, grimdark, painterly digital art, dramatic chiaroscuro lighting, muted desaturated palette with corrupted-crimson accents, weathered post-Cataclysm atmosphere, intricate high detail, cinematic composition

Huge Construct, Unaligned

CR 12SoldierDungeon / Structure

AC 20 (adamantine plate)

HP 217 (15d12 + 120)

Speed 30 ft.

STR

save +12

DEX

save +0

CON

save +13

INT

−4

save −4

WIS

save +6

CHA

−5

save −5

Resistances Cold, Fire, Lightning, Thunder

Immunities Poison, Psychic; Charmed, Exhaustion, Frightened, Paralyzed, Petrified, Poisoned, Prone, Stunned

Senses Darkvision 120 ft., Tremorsense 60 ft.; Passive Perception 11

Languages Understands its creator's language but can't speak

Traits

Adamantine Body. Critical Hits against the Adamant Sentinel count as normal hits.

Immutable Form. Immunity to any spell or effect that would alter its form.

Unstoppable March. The Adamant Sentinel's movement is unaffected by Difficult Terrain, and it can't be forcibly moved. It also ends the Grappled and Restrained conditions instantly by automatically destroying whatever effect caused them.

Guardian's Reach. Enemies within 15 feet of the Adamant Sentinel provoke an Opportunity Attack when they move away from it — the Adamant Sentinel's reach is 15 feet.

Actions

Multiattack. Makes three Adamantine Fist attacks.

Adamantine Fist. Melee Attack Roll: +12, reach 15 ft. Hit: 21 (3d8 + 7) Bludgeoning damage, and the target makes a DC 20 Strength save. Failure: The target becomes Prone and is pushed 10 feet back.

Seismic Stomp (Recharge 5–6). The Adamant Sentinel strikes the ground. Every enemy in a 30-foot Emanation makes a DC 20 Dexterity save.

Failure: 32 (5d12) Thunder damage, and the target becomes Prone and its Speed is reduced by 20 feet (until the end of its next turn).
Success: Half damage, no other effect.

Reactions

Sentinel's Wrath (3/Turn). When an enemy within 15 feet of the Adamant Sentinel does one of the following: (a) moves away from the Adamant Sentinel, (b) attacks a target other than the Adamant Sentinel, (c) tries to move past the Adamant Sentinel —

The Adamant Sentinel makes one Adamantine Fist attack. On a hit, the target's movement ends immediately, and in case (b) its attack is canceled.

How to Use

This is an impassable wall. This is exactly the piece Tier 3 was missing.

Sentinel's Wrath (3/Turn) is critical — it gives the monster three Reactions per turn. So:

Anyone trying to flee → gets hit, stops
Anyone attacking the boss → gets hit, attack canceled
Anyone trying to go around it → gets hit, stops

You can't ignore the Adamant Sentinel. You have to kill it. And it has AC 20, HP 217, Crit immunity, Resistant to most things.

Correct usage — boss protection:

Boss (e.g. a Lich, CR 21) + 2× Adamant Sentinel

The Sentinels stand in front of the Lich. The party can't reach the Lich — every attempt eats a Sentinel's Wrath.

The Lich rains spells from behind. The party has to make a choice: break the two Sentinels (434 HP total!), or find another way to reach the Lich?

How the party wins:

Teleport — Dimension Door, Misty Step. Sentinel's Wrath stops movement, not teleportation!
Flying — 15-foot reach, but limited upward.
Force damage — Force isn't on its Resistance list. Disintegrate, Bigby's Hand.
Separating — the Sentinel is slow (Speed 30). Lure it away from the boss.

Balance note: AC 20 is very high. A Lv12 Fighter (+9 attack) hits 50% of the time. This is intentional — beating the Sentinel down should be slow, so the party looks for an alternative solution.