My current villain is a Rakshasa posing as a noble woman in the town of Fallcrest. The players actually ran a job for her during their first adventure, not knowing who or what she was (they recovered a Ring of Mind Shielding for her).
The main gist of the campaign is that the devils have started their final push in the Blood War, and plan to use the Material Plane as their staging ground. A pair of old dwarf wizards discovered the plans and set their own plan in to motion, to completely seal the Material Plane off from the rest of the outer planes.
It requires casting a powerful ritual (using an arcane machine) at the exact right moment to work. The wizard cast the spell and then had to leave to enact another part of the plan, and after he left but before the machine had finished its work, the Rakshasa snuck in and dismantled the machine.
The players now are working to go back to that moment and stop whatever went wrong with the machine. They don't know she's a Rakshasa, and they don't know she was even responsible for the mishap. So it will be fun to build up to that reveal.