How do you start a rave?
Throw a flashbang into the epileptic ward of a hospital.
The joke plays on the idea of a rave, which is known for flashing lights, and an epileptic ward, where people are vulnerable to seizures due to flashing lights. The humor comes from the dark and disturbing thought of throwing a flashbang into a place where it could cause harm and chaos. It combines a fun event (rave) with a horrible act, thus creating a dark twist.