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Nehalenia's Mirrors were magical artifacts with oracular abilities. Aridea, stepmother of Renfri and Fredefalk's second wife, owned such a mirror.

...she came into possession of a rare and powerful artefact. One of Nehalenia's Mirrors. They're chiefly used by prophets and oracles because they predict the future accurately, albeit intricately. Aridea quite often turned to the Mirror— With the usual question, I take it, interrupted Geralt. "Who is the fairest of them all?" I know, Nehalenia's Mirrors are either polite or broken.
— pg(s). 86 – 87, "The Lesser Evil", in the collection The Last Wish (UK edition)

see also

In The Witcher computer game[]

In Chapter IV, Alina, a girl from the village of Murky Waters, is said to have owned such a mirror — broken. It is not clear if she broke it herself or if it was always broken. If she did break it, then why she did so, no one knows, but finding the shards and mending the mirror is part of a quest that is required to finish the game.

In the game, the artefact is referred to as a "Mirror of Nehaleni".