Depends on severity. Seizures during brain surgery are rare due to the presence of anesthesia but if a seizure did occur during a surgery, the brain pressures could increase and hence be life threatening. Most neurosurgeons are properly trained to deal with the situation but a very large seizure during surgery could be life threatening. As your physician for more details.
Unlikely. When you are under anesthesia you generally do not have seizures. If your tumor or seizure center (epilepsy focus) is near the parts of your brain that control movement or speech, you may be awake during surgery to respond to your surgeon. Your responses help your surgeon ensure that he or she treats the precise area of your brain needing surgery.