Mirror of Loss
The Mirror of Loss is a large mirror found in both the Gauntlet of Shar at X: -645 Y: -735 and the House of Grief at X: -474 Y: -1650. The former is cracked and cannot be used for reflection, however the latter is functional and bestows a bonus and malus upon the user.
Interaction
It is highly recommended for the player to prepare and save before interacting with the Mirror of Loss. To access the benefits of the mirror, one party member must first pass either an Arcana (DC 25) or Religion (DC 20) check to recognize the purpose of the mirror. Each party member may then surrender a memory to the mirror, optionally attempting a Religion (DC 25) check beforehand to offer a prayer to Shar. The character who underwent the Mapping of the Heart receives Advantage on the Religion check to offer a prayer.
Surrendering a memory to the mirror requires choosing one Ability to sacrifice, which will be temporarily cursed with Stolen Vigour, a -2 penalty to the chosen ability. The curse lasts until taking a Long Rest or using the Remove Curse spell. If the character made the successful prayer check beforehand, they might unlock a dialogue choice of a second Ability which will receive a permanent +2 bonus (up to 24). Memory dialogue choice that triggers this additional dialogue is random and not class-specific, with "incorrect" choices yielding only curse and no boon.
If you have read the Necromancy of Thay and acquired the Forbidden Knowledge condition, you may surrender it instead of an Ability. This will allow you to receive the +2 bonus even if you failed the prayer check.
You may sometimes receive the following feedback from the Narrator which will provide an additional +1 (up to 24) to Charisma called Patriar's Memory: "Disappointment floods your mind. You were sure to succeed Stelmane, but Lord Enver Gortash has stolen your support. All that charming and cajoling in the parlours of the Upper City, Wasted." This memory will occur with a degree of randomness.
You have the option of attempting to trick the mirror with a false memory (Deception DC: 20). This will result in neither curse nor boon being applied.
Shadowheart
Shadowheart is generally denied the mirror's benefits. As the game's narrator says: "With so much of your memory already surrendered to the mirror, you have nothing to offer. A sorry outcome." As such, she is not given the chance to be cursed nor booned.
If she is no longer loyal to Shar she has disadvantage on the 
Religion check to offer a prayer to the mirror. Passing this difficult check ultimately still yields the above narration.
However, if Shadowheart becomes a Dark Justiciar, she can receive a permanent +2 bonus to any Ability of her choosing without sacrificing an Ability. Possibly, one has to do strictly everything Shar has asked of her (killing the Nightsong, killing Viconia and killing her parents).
Becoming a Dark Justiciar gives her an advantage on the Religion check to offer a prayer to the mirror. Passing it does not seem to do anything, as is not required to get the +2 bonus to Ability Scores.
Strategy
Only one character needs to pass the initial Arcana or Religion checks to identity the mirror, but each party member must pass their individual Religion prayer checks in order to receive the permanent ability buff. Items that boost Religion or Intelligence, which can be passed to each party member for each attempt, will skew the odds in your favor. These include:
Warped Headband of Intellect
Mage's Friend
Shapeshifter's Boon Ring (when disguised or shapeshifted)
General spell and class-specific buffs to checks like Guidance, Enhance Ability and Bardic Inspiration additionally shine here.
Alternative to having the Warped Headband of Intellect equipped, the character can be temporarily respecced through Withers, giving them an Intelligence rating of 17 that can be further increased with the ability improvement feat twice, to 20. The character can be respecced back to their main build afterwards, retaining the Mirror of Loss stat enhancement.
Notes
- You may choose to sacrifice an ability with at least base 8 (see picture).
- More than one character may gain the same ability improvement (As of 2023-Nov-05, Tav and a hireling were both able to gain +2 Cha.).