I want an invading army from beyond this dimension. Tell me about it.

[Mathew_Freeman]

Keeping with the 'invading humans' theme, invasion from a Mirror Universe, Star Trek style, might be fun. That way you get to use the PCs as enemy NPCs!

...and their friends, allies, cohorts and familiars, not to mention using twisted verisions of their own tactics and equipment...

 

[DMH]

Or have PCs from the future leading the army- they know everything that is going to happen and are trying to stop themselves from whatever.