Repair kit

Mire repair kits, also referred to as immortality charges, were battlefield medical kits found in Mire helmets. These chips, biologically linking with their patients, continually repaired them.

The Twelfth Doctor modified two for human use, and placed one on the head of a dead Viking girl named Ashildr, resulting in her resurrection and subsequent immortality. The charge never stopped repairing her. The Doctor also left her with the kit, which she could use on anyone she'd decide she didn't want to live without. (TV: The Girl Who Died)

Ashildr kept the extra kit with her person, as a necklace. She told the Doctor she never found anyone worthy enough to give the kit to. The second immortality charge was eventually used on highwayman Sam Swift by Ashildr to close a portal to Delta Leonis, and stop a Leonian invasion in its tracks. Due to the power drain from the portal, though the Doctor was uncertain whether or not Sam Swift was immortal, too. (TV: The Woman Who Lived)

Because of the restoration chip, Ashildr's immortality remained for billions of years, up to the last 5 minutes of the universe, outliving every other immortal. (TV: Hell Bent)