This item appears in Baldur's Gate, Baldur's Gate: Enhanced Edition and Baldur's Gate II: Shadows of Amn. In Baldur's Gate, it's one of the best shields in the game, along with the Large Shield +2. It's gained as a reward from the A matter of life or death side quest.
In Baldur's Gate II, it can be found in the room with the furnaces in the power core of the sphere. It looks no longer the same as in Baldur's Gate, but like any other large shield.
|“||Ilmater himself enchanted this shield upon seeing a young squire's willingness to endure untold agony for the well-being of his mount. During a fierce battle between a human cavalry division and a hobgoblin horde, the young man was sent for reinforcements. He was beset by a cascade of arrows that nearly blocked out the sun, and he knew that even if he avoided the arrows by himself, should his horse fall he would surely be doomed. With his shield and body he positioned himself to take brunt of the onslaught, and was mortally wounded for it. His horse remained unscathed, allowing for their escape, and his body was carried back to the nearest town, where he was resurrected from death.||”|