I think it's borked. It will often say that {some insane number} of units has perished. Commander (even sneaky seems to usually die in those cases). However, sneaky units doesn't seem to suffer much.
Stygian Paths was bugged earlier. It hopefully works better now.
There is no battle. Some units take dmg and others are targeted by a MR attempt on their lives IIRC. Undead are not immune IIRC. The lord of the underworld takes back what is rightfully his.
1) Are demons immune to "Darkness"?
2) I tried windriding one of my harbingers out of a province but it said it was too big and heavy. But harbingers are only size 4. So is there some randomness to the allowed size?