in the background you can see a "Dark Bugler"
he spat them out at the 6 person party of 75, just two of them made mincemeat of them. sounds fun...

the people who saw it didnt claim it because it ran by while they were fighting, although it probably would have torn them to pieces. he was gone after that... but he left his little imp friends for them to play with.

I don't think you actually have to fight the Dark Rider to do the mission. I was out killing stuff with some people in my linkshell last night before besieged, and this thing spawned right after we killed another NM (Dark Esquire). I don't know if he always spawns after the Dark Esquire, but it seemed at the time like that was what happened. Anyway, the Dark Rider didn't stay around for very long. He despawned when nobody claimed him, and left a hoofprint where he despawned. I'm thinking all you need to do to complete the mission is just click on the hoofprint. Any it stays up for a long time (We stayed another hour to see if either or the two NMs would spawn again, and the hoofprint was up the whole time). Just a note, the hoofprint was *not* there before he spawned. The spawn location was K-9 in wajaom woodlands.