The Mirror of Loss in BG3 is a giant mirror found in the Gauntlet of Shar and the House of Grief with no reflection. While the former is cracked, the latter is usable and can be found in the Lower City in the Chamber of Loss in Act 3.
How to Use the Mirror of Loss | Baldur's Gate 3
In this guide, we'll look at how to use this Mirror of Loss in Baldur's Gate 3. This magical artifact will help you improve your Ability Score stats, but only after you give up some of your memories.
What the Mirror of Loss in the House of Grief does exactly is that it gives you a chance to lower 2 points in one stat (by giving it a curse) and raise 2 points in another stat. The curse can be removed, so essentially the Mirror of Loss can be used to get a permanent stat boost without losing much.
When you come across the purple mirror, save your game first. This is crucial because you only have one chance (per character) and you cannot fail if you want this bonus.
