Repair kit

The Mire medical kit was a tool used by the Mire to repair themselves if necessary after a battle. After the a battle with the Mire in a Viking village, the Twelfth Doctor used a part of the kit to resurrect Ashildr, who had been killed during the battle. The kit brought her back to life and would never stop repairing her, making her effectively immortal. (TV: The Girl Who Died)