Based on the described behavior, it seems that Nathaniel's friend is likely experiencing a reaction to a hallucinogenic substance. The mention of swinging a knife and talking about fireflies attacking him suggests that he may be experiencing hallucinations or delusions, which are often associated with drugs like LSD (lysergic acid diethylamide), psilocybin (magic mushrooms), or other psychedelics.
Additionally, if the drug caused extreme agitation and violent behavior, stimulants such as methamphetamine or certain synthetic cannabinoids (often referred to as "spice" or "K2") could also be involved, as these can sometimes lead to aggressive or paranoid behavior.
However, hallucinogens would be the most likely category given the description of the behavior. It’s crucial to emphasize that anyone displaying such symptoms should be treated as a medical emergency, and professional assistance should be sought immediately.