[ Memory Mixing ]
As mentioned in canon lore, Vaermina collects the memories of sleeping people by reaching out into Mundus from Quagmire. She also collects dreams (and therefore memories) via the Skull of Corruption and may additionally have a way to access the memories of Skin-Stealers, given the potential link she has to them.
Collection is not where the Prince stops, however. Just as much as Vaermina has the potential to steal memories, she also has the ability to put them back.
That said, it’s not that simple. The memories Vaermina places back may or may not be the ones she directly stole — At the very least, aspects of them might be twisted slightly, in order to scare and unnerve a chosen target. Where she becomes truly terrifying is when she starts replacing entire swathes of memories with those from other people, tailoring them and twisting them to believable (if unsettling) visions and utterly convincing the victim that they are their own.
This is exactly why Mirthys does not truly know why she’s in the Fourth Era. She has a vague recollection of what transpired, but it is missing several pieces, and she’s well aware that those memories might not even be hers in the first instance. All she can do is take those precious scraps as a founding truth, and attempt to build the bridges as best she can.
Vaermina’s not going to make that search easy. For every connection and recollection made, another is corrupted as punishment for defiance. As much as Mir would like to think she will remember her newest Fourth Era memories and friends no matter what may come, nothing is safe…