I'm pretty sure it never actually materialized. You can ping Black Blade Publishing to confirm.
... and yes, Cairn goes very well with Barrowmaze, btw. In my campaign, Cairn is a much larger barrow to the north of and easily visible from the Barrowmaze area. My players haven't braved Cairn yet, because they're finding Barrowmaze very lucrative. Deadly too (3 PCs and 2 men-at-arms dead so far), but that hasn't dissuaded them.
Yeah, I thout it mit be a great side adventure. Too bad I can't get my hands on a copy. Thanks for the note though.