I spent 2 nights about a week ago in Halls of Betrayal, and had quite a good time. First time I'd actually dungeon crawled in a long time, so it was a nice change of pace.
Anyhow, the mask drops off A Sarnak Champion. Looks like a regular, live Sarnak with the red chest piece. Hits for 300ish, slowable. From what I've seen on Magelo, it is a 31% haste. We were on the backside of the castle, not far from the zout, when we got it. Camping there for 2.5 hours, we saw 3 Champion spawns, our group got 2 of them. The other look was some 20str Cloak, can't recall the name.
Just looted one of these the other night. It's a normal "live" sarnak. From what I've seen any of these rare sarnaks that drop loot in that castle area can spawn in any of the locations. I've seen them in so many different areas that I think each spawn spot can be a rare.
The other named undead versions roam ALL over, so you can't really camp a specific spot either on them.
Crawling the castle is about the best way I've seen to really hit all the rare mobs that you possibly can get.