- Parasite (The Faculty)
After her ocean planet dried up, an unknown alien parasite queen made her way to earth and took over a small high school by infecting the staff with her tadpole like offspring. The infected can survive being cut into pieces, gunshots to the head, they can reattach lost limbs including their head and can seemingly shapeshift. Their queen is a huge, hulking, tentacled monstrosity straight out of a H.P. Lovecraft story.
Attempting to take over the planet by starting out small makes sense. Why go for the White House when you can just as easily assimilate a school and spread out from there. Their only weakness appears to be that if you take out the queen, the rest will perish. Whether or not more queens can be created is never revealed but I would assume that to be the case. I doubt the species would keep all its eggs in just one tentacle laden basket.