The altercation happened at an apartment in East Lampeter Township on Monday. Police say 32-year-old Nicole Marie Wagner used the spear to stab her 31-year-old female roommate and threatened to kill her.
Township police Lt. Robin Weaver says the victim was stabbed in the back and hand and was treated at the hospital.
Police say Wagner had accused her of wasting electricity by not turning off lights.
Wagner was being held on $50,000 bail in Lancaster County Prison. She faces charges of aggravated assault and terroristic threats. It wasn't immediately clear if she had an attorney.