Helmet of Determined Will
Wondrous item, rare (requires attunement)
This helmet activates whenever its wearer would be charmed, frightened, or otherwise mentally coerced. The wearer becomes immune to being charmed or frightened, and to any effects that would force it to take an action or move against its will, though it does not protect against physical push or pull effects. This immunity lasts for 10 minutes. The helmet can activate multiple times during the same day, but each time after the first, the wearer suffers one level of exhaustion. The helmet’s activation is not a choice. If its conditions are met it activates, and the wearer must deal with the consequences.
Wearing the helmet for an extended period exerts a mental toll as well. When the creature attunes to the helmet it loses one of its memories. Each day at dawn, if the creature is still attuned to the helmet it loses another memory. The DM and player should work together to make a poignant scene about the cost of using this powerful item.
Ref: T&H p61