The Ark

[Tifone]

A spy discovered in enemy territory and retreating AFAIK he dies very often, not being able to reach a friendly province. So you could use a spy like this near friendly provinces but it would look like a waste to me, you could just raise a temple nearby ^_^ For the "deep in enemy territory" part, AFAIK the effect of Dark Skies happens just in your dominion.

The Ermor/Oni point I don't understand ^_^ Having a large army of udneads and demons isn't the most probable thing to happen with many nations... and if the Ark damages your troops, it's obviously a wrong occasion to use it - like everything else in the game, you use this item and tactic in the right moment and occasion. If my armies are just undeads and demons, I wouldn't even bother forging the Ark if not to keep my enemies hands away from it ^_^