Phylactery is a quest item in The Witcher 3: Wild Hunt, automatically obtained at the end of the main quest Get Junior.
History[]
This item was first used by Eredin, who released a powerful curse from it to turn Avallac'h into Uma. However, the nature of the spell took some time to actually manifest and so Ciri took the now broken item with her to Novigrad, where she found Dandelion and asked for his help in repairing it, in hopes of lifting the curse.
While the bard didn't know how to fix it, he figured Whoreson Junior could and so they took it to him. However, the criminal leader demanded a steep price to fix it: for them to steal Sigi Reuven's closely guarded treasure. They left the broken phylactery with Whoreson Junior while they made plans to steal the treasure.
The plan went wrong though and they never returned. Geralt, knowing Junior knew something about the two, assaulted the criminal in his house, who eventually mentioned the deal he'd made with them and Geralt took the phylactery with him. When he ran into Yennefer later in Vizima, he handed it off to her for safe keeping.
At Kaer Morhen, Yennefer informed Geralt that it needed to be charged, so he and Lambert set out to the Circle of Elements and re-charged it. When they then began the Trial of the Grasses on Uma, Yennefer was able to reverse the spell and had it re-captured back into the phylactery.