Ancient Enemy

The Ancient Enemy is the main antagonist in the Dean Koontz novel Phantoms, later adapted into a film. It is a shape-shifting monster that lives underneath the fictional town of Snowfield, Colorado.

Similar to Pennywise the Clown from Stephen King's IT, the Ancient Enemy can transform into nearly anything and makes its home in the sewers. It is similar to a large amoeba and can send "phantoms" of itself to hunt prey, absorbing the knowledge and memories of the people and animals that are consumed. It believes itself to be Satan and is speculated to have caused the extinction of the dinosaurs and mass vanishings throughout history.

The Ancient Enemy's cells are similar to fossil fuel, which enables the protagonists to defeat it via oil-eating bacteria.