Invincible Cuirass

Armor, legendary (requires attunement)

This elaborately filigreed +2 halfplate always remains beautiful and shining. It is impervious to rust (including rust monsters and similar effects) and cannot be damaged by acid, cold, fire, lightning, or thunder. This does not grant the wearer this immunity, but it does reduce any acid, cold, fire, lightning, or thunder damage taken by the wearer by 5 points (minimum 0).

The wearer can alter the structure of the armor’s layers beneath the cuirass as a reaction or bonus action, forming it into overlapping metal scales (granting resistance to bludgeoning damage), layered chain rings (resistance to slashing), or angled plates (resistance to piercing). This resistance applies even against magical attacks but not against attacks with adamantine weapons.

Finally, whenever an attack against the wearer would be a critical hit, roll 1d20; on a roll of 11 or greater, the attack is resolved as a normal hit instead.