1. Wipe the memories of everyone on a planet. Let them know how to talk, know the fact that they must eat, etc., but no one knows who they are, or how to use magic, technology, etc. Real confusion.
2. Destroy all standing water.
3. Increase gravity 10 fold.
4. Decrease gravity 10 fold.
5. Destroy the atmosphere.
6. Pull a Tower of Babel on them, and remove language from their minds.
7. Destroy their white blood cells.
8. Turn the entire population (including animals) into vampires. Nothing to feed on...