Just in case anyone's curious, I based my assumptions about nether and aether in that last post on them being alternate terms for Skallagrim's description of galdhr and seidhr. Which would be why Arty can't sense undead, because they'd be powered by galdhr (nether) while he's powered by seidhr (aether).

I'm also assuming that we're two floors below the Mystan elder (I think that's where Melon put us...) and that it's just undead guards on the next floor up. And that the elder isn't directly above us, since twenty meters is about sixty five feet.