There was a bug in the stygian paths prior to 2.10 that made it useless. I'm not sure if it has been much tested lately.
9.3 mil seems like a bug, but dying regardless of hit points is not strange. Everyone using the paths has a slight chance of being caught by the lord of the underworld. Hit points does not protect you from being kept in the underworld.
In 2.14, I used it three times in one turn with two demiliches and my pretender. I believe everyone was alone. One demilich died (I think he was moving into enemy territory) and each casting said something like 50,000+ units died.