Ghostfence plot:

The year is 3E 407, and the Tribunal has ordered the construction of the ghostfence to keep the blight and Dagoth Ur's minions at bay. A few months into the project, attacks on construction crews by ash vampires and the like have become so common that Ordinators, Temple associates, Armigers, and the various helpers from the Great Houses are no longer enough to keep the project going on schedule. In addition, scavenging crews responsible for obtaining building materials from the ashlands have been unable to operate for the same reason. Because of pressure from his superiors, the commander at Ghostgate (who i'm too lazy to name at the moment) is forced to hire out mercenaries and mercenary groups from all over Tamriel. (that frees up the races so we aren't limited to dunmer.)

Now here's the twist. As we play, the commander will send people on various missions that aren't outright suspicious, but don't really make sense either. Eventually, the commander is found to be a Sleeper working for Dagoth Ur, but being mercenaries with no pull in Vvardenfell politics, the players are forced to deal with the problem themselves.