That'll be Ophiocordyceps unilateralis. It invades the ant's brain and causes it to leave it's nest and go somewhere better for the fungus then wait to die as the fungus errupts from its head.

If we're talking about nightmare mind control horrors, we shouldn't forget our old friend toxiplasmosis gondii, which infects rodents, then alters their behaviour so they're not afraid of cats, in particular. This leads to the rodent getting eaten so the parasite can infect the cat, which is the only place it can reproduce, before spreading from the cat faeces back into the rodent population. It can also infect humans where there is evidence that it affects behavior too, particularly making males more careless of rules.

Sleep well.