The Queen's Domain Memento Mirror is handed over by the Herald of Okbujo, the boss in the Record of Sin, and it is an infinite use item whose hint reads 'Seek the souls you once knew'. It is used on the ghosts of characters who have passed on, and each ghost gives up a memento only after you have finished their quest.

Getting the Queen's Domain Memento Mirror

The Herald of Okbujo is fought in the Record of Sin library, and its achievement is Dread Silver. Talk to the Herald after the fight and it hands the mirror over, rather than dropping it on the ground.

The boss guide has the fight with the Herald of Okbujo.

The eight ghosts

Each ghost leaves a memento only once you have finished their quest and they have moved on. A ghost that gives you a reward but no memento means you are not done with that person yet.

GhostWhat you must finish first
Chef Gagi60 meal tickets
The KingYour father, who is the reason you came to the island
Commander EkontaThe knight who gave you a sword at the very start
Captain Aganmi of the Iryupe RaidersHis crew's necklaces
Erma, Matron of the DeepEvery Orbewa egg
Mechanical Engineer OssiAll of his modules
Knowledge Keeper WourhaA full bestiary
PehaFound at her coffin; she gave you the Puwema lens

Wourha is the slowest of the eight because her requirement is a complete bestiary, and the Royal Orbewa eggs and the meal tickets both take a full sweep of the island. Doing them as you explore rather than in one run is what makes this list finishable.

The bestiary guide has the enemies each ghost asks you to hunt.

The altar in Deimpo Mausoleum

The eight mementos go to a hidden altar in Deimpo Mausoleum: a room with four symbols on the wall at the bottom of the big red water room that holds a Wind Gem, behind a door that needs all four lenses of the Mask of Second Sight. A second way in runs from below the Tower of Knowledge near the prison.

Most players reach it with Akunra, but the Cestus uppercut as an extra jump works as well, and so does flight. The altar forges Okbujo's Judgement, the sword tied to the third ending.

The Petrified Garden seals guide has the lenses the door needs.

What the mirror is for

Okbujo's Judgement is the weapon that carries the third ending, and beating Queen Siraatsu with it for the whole fight and choosing Banish at the end is what turns the run into that ending. Getting the mirror and the eight mementos is therefore a long side quest that happens to be the backbone of the endgame.

The endings guide covers all four endings.