Long ago, this artifact belongs to an ancient elf lord Gilmarra, used to send prisoners into exile as punishment.
Centuries later, the Scion of Orcus obtained this mirror and was kept at Castle Greywall, though the details on how he obtained this is unknown. After the castle was destroyed, Sir Razalaxx sent an excavation to retrieve the mirror, not because he desires it but so that Lady Sariel doesn't. The night after the mirror was found, several ghouls under Lady Sariel attempted to steal the mirror but was unsuccessful. The very next day, some duergar stole the mirror (as well as Razalaxx and Edric) as part of the aboleth's plot to dominate the world.
In the World Below, the mirror was used as a component of the Focusing Lens, allowing the domination effect broadcasted from the Tower of Domination to be more efficient; having the effect last longer and be used more frequently. The Shield of Gravesford, later on, was able to find the Focusing Lens and destroy it, although the fate of the Mirror of Gilmarra is left uncertain.
The Mirror of Gilmarra is nine feet by foot feet, and made to look like two trees growing together.
It can be used to allow the viewer to see as per the scrying spell with detect thoughts as per a Crystal Ball of Mind Reading, but it also allows the viewer to see into other times, both past and potential futures, and use the mirror to journey there.
If someone were to use the identify spell on it, it will only reveal it as a powerful artifact that can be used to see into another place, or even another time, similar to a crystal ball.