[Endoperez]

The army of the dead would actually be more akin to a ritual spell, or perhaps an event or site effect. Those aren't easy to mod, though. Replacing Ghost Riders might do it.

Anything besides nations is almost impossible to do, though, because almost everything was "pre-scripted", unique, and needed special condition to trigger. As an example, the nine would be dead once they died. The only way to model anything like them in Dominions would be unique summon spell, which would be expensive but could be casted once/turn if the player hadn't spent too much resources during the first 30-50 turns.