Shield Guardian

The monster is magically bound to an amulet. While the monster and its amulet are on the same plane of existence, the amulet's wearer can telepathically call the monster to travel to it, and the monster knows the distance and direction to the amulet. If the monster is within 60 feet of the amulet's wearer, half of any damage the wearer takes (round up) is transferred to the monster.

The monster regains Hit Points at the start of each of its turns if it has at least 1 Hit Point.

A spellcaster who wears the monster's amulet can cause the monster to store one spell of level 4 or lower. To do so, the wearer must cast the spell on the monster while within 5 feet of it. The spell has no effect but is stored within the monster. Any previously stored spell is lost when a new spell is stored. The monster can cast the spell stored with any parameters set by the original caster, requiring no spell components and using the caster's spellcasting ability. The stored spell is then lost.

The monster makes two attacks.

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Trigger: An attack roll hits the wearer of the monster's amulet while the wearer is within 5 feet of the monster.
Response: The wearer gains a +5 bonus to AC, including against the triggering attack and possibly causing it to miss, until the start of the monster's next turn.