Source: Baldur's Gate - Descent into Avernus

Armor (shield), legendary (requires attunement)

The Shield of the Hidden Lord is of celestial origin and serves as a prison for the pit fiend Gargauth, whose mortal followers revere it as a god. Over time, Gargauth's evil has warped the shield's appearance, so that its celestial motif and designs have become twisted into a fiendish face that subtly moves in disturbing ways.

While holding this shield, you gain a +2 bonus to AC and resistance to fire damage.

Sentience. The Shield of the Hidden Lord is sentient as long as it imprisons Gargauth. While sentient, the shield has the following properties:

• The shield has an Intelligence of 22, a Wisdom of 18, and a Charisma of 24, as well as hearing and truesight out to a range of 120 feet.
• The shield can speak, read, and understand Common and Infernal, and it can communicate telepathically with any creature it can sense within 120 feet of it. Its voice is a deep, hollow whisper.
• The shield has 3 charges. You can use an action to expend 1 charge to cast Fireball or 2 charges to cast Wall of Fire from the shield (save DC 21 for each). The wall of fire spell lasts for 1 minute (no concentration required). The shield regains all expended charges daily at dawn.
• Anytime during your turn, the shield can choose to radiate an aura of dread for 1 minute. (This is not a power of the shield that you control.) Any creature hostile to you that starts its turn within 20 feet of the shield must make a DC 18 Wisdom saving throw. On a failed save, the creature is frightened until the start of its next turn. On a successful save, the creature is immune to this power of the shield for the next 24 hours. Once the shield uses this power, it can't use it again until the next dawn.

Gargauth's Personality. Gargauth desperately seeks freedom but can't escape on its own. The pit fiend won't reveal its true nature, referring to itself only as the Hidden Lord. It drops hints and subtle suggestions that it is an angel trapped in an unholy prison. If released from the shield, the pit fiend honors the terms of whatever bargain it struck to facilitate its escape.

While trapped in the shield, Gargauth carefully steers the shield's current owner toward committing acts of cruelty and domination, hoping to condemn the individual's soul to the Nine Hells. Conflict occurs if the shield's owner does anything that would make it more difficult for Gargauth to escape its prison, such as leaving the shield in a place where others are unlikely to find it.

Gargauth doesn't know how to escape from the shield. The pit fiend believes (incorrectly) that it can break free of the shield if it's brought to the Nine Hells, for it's convinced that the shield's powers are weaker there.

Freeing Gargauth. Casting Dispel Evil and Good on the shield has a 1 percent chance of freeing the pit fiend, or automatically succeeds if the spell is cast by a solar, a planetar, or an archdevil. A god can release the pit fiend by touching the shield and speaking Gargauth's name. When released, Gargauth appears in a random, unoccupied space as close to the shield as possible.