Mnemonic Sentinel

large construct, neutral

Armor Class: 18 (Natural Armor)

Hit Points: 150 ()

Speed: 30 ft.

STR18(+4)DEX14(+2)CON14(+2)INT24(+7)WIS12(+1)CHA10(+0)

Saving Throws: INT +12, WIS +6

Skills: Perception +6, Investigation +15

Damage Resistances: Bludgeoning, Piercing, and Slashing from non-magical attacks

Damage Immunities: Poison, Psychic

Condition Immunities: Charmed, Exhaustion, Frightened, Paralyzed, Poisoned

Senses: Darkvision 60 ft., Passive Perception 16

Languages: Understands all languages but does not speak

Challenge: 12 (8,400 XP)

Adaptive Learning: The Mnemonic Sentinel learns from its attackers. The first time a creature uses an attack, spell, or ability against the sentinel, it functions as normal. The second time, the creature has disadvantage on the attack roll or the sentinel has advantage on its saving throw. The third time, the sentinel gains resistance to the damage. The fourth time and beyond, the sentinel becomes immune to the attack, spell, or ability.

Immutable Form: The sentinel is immune to any spell or effect that would alter its form.

Actions

Multiattack: The sentinel makes two melee attacks.

Slam: Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 18 () bludgeoning damage.

Energy Beam (Recharge 5-6): The sentinel fires beams of concentrated energy. It can target up to 3 different creatures within 60 feet, but it cannot direct more than one beam at a single target. Each beam requires a separate ranged spell attack: +10 to hit, range 60 ft. On a hit, the target takes 18 () force damage.

Reactions

Mnemonic Recall: When the Mnemonic Sentinel is targeted by a spell, it can use its reaction to analyze the magical effect. If the sentinel makes an Investigation check (DC equal to the spell's level + 10), it learns the spell and can cast it once without requiring components. The sentinel must use this stored spell before the end of it's next turn, or it loses the knowledge.

Notes

The Mnemonic Sentinel is a towering construct of uncanny design, forged from an ancient metal that shimmers with an otherworldly sheen. Its limbs and torso are covered in runic inscriptions that pulse faintly when it moves, suggesting a powerful magic core at its center. Where its face should be, there is a runecarved plate, with only a single glowing glyph serving as an eye.
The sentinel’s most dangerous ability lies in its perfect memory. It records every move and tactic used against it, quickly adapting to render those same strategies ineffective. With each attack, spell, or ability directed toward it, the sentinel learns, adjusts, and overcomes, making it nearly invincible to repetitive tactics.
In combat, the Mnemonic Sentinel is methodical and unfeeling, calculating its movements with the precision of a mind far superior to mortal intellect. It relies on its massive frame to deliver punishing blows, but when threatened from afar, it unleashes devastating beams of pure energy. Once it learns a new spell or magical effect, it can even replicate it for a short time, turning its enemies’ own powers against them.
Whether created as a guardian for some long-forgotten civilization or as a weapon in a war of ancient magic, the Mnemonic Sentinel is a fearsome relic of an era when machines were made to rival the gods themselves.

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Tags:forceactionreactionresistanceimmunityconstructsaving throwspells