This is the only fic I can think of where Tara is possessed.
Tom Cat by jamesmarstersjunkie [NC-17]
Summary: THis is a story set in season 5 and was inspired by my cat Temple and is one that I've been wanting to write for a while now. Ive always loved Egypitian mythology and so in this story we see an being eons old testing Tara's purity while Buffy is finding it difficult to grasp the concept of Shakespearean poetry. Will Tara find her strength? Will Buffy find a tutor who won't make her feel stupid and when she does, will she like him? She will when she finally gets to know him...or should I say, the man he used to be? The gods only know.

Oh and after I posted this story I realized that I hit the wrong season. THis is season 5...sorry.
Categories: Season 4
http://dark-solace.org/elysian/viewstory.php?sid=3326