I was reading this answer to a question What is Petyr Baelish's Endgame with Sansa?
Now in it they mention
"With Catelyn dead (or undead, as it turns out)"
Now I don't recollect any where from the books that it mentions
she is undead
Can someone please clarify this for me?
(I have read all the books and watch the TV series, but please put spoiler tags in for other people)