Little Box of Horrors - S7-E18
Corrected entry: Katya kills Nina and then takes Nina's form when she visits the Charmed Ones. But why would Katya need to take Nina's form? The Charmed Ones have never seen Nina before, nor did they know anything about her, so she could have just shown up as herself, but maybe not wearing the red leather body armor. Also, why would Katya give the Charmed Ones so much accurate information about herself and about Pandora's Box? The whole setup was done to fool the viewers, but made no sense in terms of trying to fool the Charmed Ones.
Correction: Katya can't know for sure that they've never met Nina. Also, Katya knows that the Charmed Ones have the book which often has pictures of the demons in it and may have seen her (or Nina) while flipping through it. Even though in this case, there is not a picture of her next to her description in the book, she is safer to assume that there is.
Myridon