A Watcher Most Charmed by toots
http://www.tthfanfic.org/Story-16752/toots+A+Watcher+Most+Charmed.htm

Awesome series that really showcases Xander and shows what would happen if the slayers were present in Harry Potter and Charmed with some characters from Stargate and The Mummy. In the case of the Charmed ones Pru is called as a slayer which saves her from dying, Tonks is called which saves Harry, Xander is related to O'Niell which saves the clone, there is even some Highlander tossed in.  He is even able to rescue Cordelia. I like Malfoy in this story because while he is a little snot he is an honorable one and shows that by his actions. The story is well paced with likeable characters, there is action, romance, prophecy, gods, special children and the end of the world. The story is long and well worth rereading.