What is trepanation? why was this used?

Trepanning, also known as trepanation, trephination, trephining or making a burr hole (the verb trepan derives from Old French via Medieval Latin, from the Greek noun of trypanon, literally "borer, auger") is a surgical intervention in which a hole is drilled or scraped into the human skull, 

Trepanning, also known as trepanation, trephination, trephining or making a burr hole (the verb trepan derives from Old French via Medieval Latin, from the Greek noun of trypanon, literally "borer, auger")[1][2] is a surgical intervention in which a hole is drilled or scraped into the human skull, exposing the dura mater to treat health problems related to intracranial diseases or release pressured blood buildup from an injury. It may also refer to any "burr" hole created through other body surfaces, including nail beds. It is often used to relieve pressure beneath a surface. A trephine is an instrument used for cutting out a round piece of skull bone.

In ancient times, holes were drilled into a person who was behaving in what was considered an abnormal way to let out what they believed were evil spirits.[3] Evidence of trepanation has been found in prehistoric human remains from Neolithic times onward. Cave paintings indicate that people believed the practice would cure epileptic seizures, migraines, and mental disorders.[4] The bone that was trepanned was kept by the prehistoric people and may have been worn as a charm to keep evil spirits away. Evidence also suggests that trepanation was primitive emergency surgery after head wounds[5] to remove shattered bits of bone from a fractured skull and clean out the blood that often pools under the skull after a blow to the head, which is called a subdural hematoma. Such injuries were typical for primitive weaponry such as slings and war clubs.[citation needed] There is some contemporary use of the term. In modern eye surgery, a trephine instrument is used in corneal transplant surgery. The procedure of drilling a hole through a fingernail or toenail is also known as trephination. It is performed by a physician or surgeon to relieve the pain associated with a subungual hematoma (blood under the nail); a small amount of blood is expressed through the hole and the pain associated with the pressure is partially alleviated.