What does sectumesempra do?

Sectumsempra is a spell from the Harry Potter series, created by Severus Snape. It is used to inflict deep cuts or wounds on the target, functioning similarly to a slashing attack. The spell can be quite dangerous and is typically considered dark magic due to its harmful effects. In the story, Harry Potter uses it without fully understanding its consequences, resulting in severe injuries to Draco Malfoy.