I'm almost positive I read it before 2009. I didn't know much about Highlander at the time, and I remember I had to look up Richie's picture to see who he was. This fic got me into reading Highlander.
What I'm fairly certain of:
-Richie and Methos are in the fic, and I remember a scene where Richie has sex with someone in a room at the mansion (showersex?) - I'm thinking another male character.
-Xander is just learning he's immortal.
-Angel not only gives Xander a really cool and old sword, but he helps Xander learn how to use it.
-The AtS/BtVS and Highlander crews stay at the mansion in Sunnydale while Xander trains and they figure out what to do about a major plot point.
-It's pretty long.
What I might be mixing up with other fics:
-Methos and Spike meeting up for a one night stand (may have known each other before), and having sex in Spike's crypt. I'm pretty sure there's another fic where Methos and Spike have a quickie, but it seems to me that the fic I'm currently searching for had this as the beginning/opening scene.
-The opening scene may also be Richie killing another immortal, but the Methos/Spike bit would be pretty close to the beginning too.
I really don't think it's Immortal Convergence by Kay, and that's usually what the fic searches result in.
The characterization was good in this fic, and I really liked the way the author wrote Spike and Xander's relationship/interactions.
I'd really like to find out what this fic is even if it's no longer online. It would be great to re-read, but just knowing what it was would be cool, and I might even have it on an old HDD, so a title and author would be a big help.