Just got one today after about 7 hours total spent killing goblins, especially the ones in the interrogator/Warlord Skargus room. When the Canyoneer spawned, he spawned in the spot closest to the Warlord's spot -- adjacent to the interrogator. Finally, now my warrior can double as group slower! Lol.
I just wanted to add that it dropped for me on July 4th, 2005. As noted, a goblin canyoneer was up in the same room where the goblin traitor spawns and the angry goblin roams from time to time. I saw the pike in his hand, so maybe you always see it if he is carrying it. It looks exactly like a wurmslayer.
In all, I did about five two-hour camping sessions in that cave and checked out that cave about twice a week for a few months before I saw the canyoneer. He is not named but he is definately rare.
I got one of these for my warrior and it is primary only, not secondary like it says above. Used it to smack around a couple of the other gobbies in the room, but didn't notice it proc at all...my dex was around 130 or so at the time so that probably had something to do with it. Gotta go try it with full buffs and check out how well it works and what lvl mob it works up to.
I'm sure you've battled a shaman of some kind that has cast a slow spell on yourself or other party members, that's what this spell does, slows the target a certain percentage.