Riddick some finds himself in the past/Buffy universe. It's up to the author to figure out how; perhaps a wish that went wrong.

While exploring the world he finds himself in, Riddick discover the most intoxicating scent he has ever smelled. So overcome by the need for the scent that Riddick allows his desire to possess the source of the smell overcome his normal survival instants. Riddick steals the source of the smell, a young man with brown hair and eyes from off the street and take it to a 'newly' vacant home.

In their new home Riddick strips the young man of his clothes and makes its so that he can not escape while he trains the young man to accept his touch.

Note: If you haven't figure it out yet the young man is Xander.

I can't see Riddick as the raping type so I image he's probably blindfolded Xander and is feeding him by hand, bathing him, sleeping next to him. Stockholm syndrome.

I also figure that this happens during a summer that way you don't have to worry about Buffy or Willow interfering since they're out of town. Pre-graduation while Buffy was out of town and Willow was at camp or post they could think he's on his road trip.