[Scott Hebert]

Broken Empire Ermor is actually one of my favorite Nation/Theme combinations. Their Grand Thaumaturgs can give them a very good chaff screen (and a Grand Thaumaturg prophet gets Longdead Horsemen). Their national units are basically Pythium's (or should I say the reverse?), which are among the best national units in the game. And, as people have mentioned, their Thaumaturgs are excellent Nether engines later.

Another note is that Death and Astral, IIRC, are the two paths that have a 1-path search spell. This allows your Thaumaturgs to search for the path sites, freeing your Grand Thaumaturgs for doing other things (like Reanimating Longdead or casting other spells).

As a final note, BE Ermor is the only nation/theme that has recruitable Unholy Priests (in any form) that does not suffer from a death scale or other drawbacks (Desert Tombs).