YW. In this one they try to fight the possession.
Unconrtollable by sweetprincipale
http://www.fanfiction.net/s/7464100/1/UncontrollableSet during S. 4 "Where the Wild Things Are". Buffy and Spike end up being trapped in the Lowell House during the peak of the sexually driven "manifestation." Short Smutty Spuffy. Something not too heavy, just read and enjoy.
The Sequel
Unmentionable by sweetprincipale
http://www.fanfiction.net/s/7509661/1/UnmentionableA sequel to Uncontrollable.S.4 Buffy and Spike were trapped in the sexual atmosphere of the Lowell House during Where the Wild Things Are. After their night of possessed passion they see each other in a different light- but refuse to share the secret.