A. K. McDowell a.k.a. Dark Evangel is a 19th-century dark witch.
She is able to cast the Killing Curse (canon) many times in a fight by casting it with indifference rather than hate.
On her first public appearance, she means to introduce herself as the Walking Catastrophe and the Apostle of Darkness but in the excitement of the moment she calls herself the Apostrophe of Darkness instead. In the 1970s, this inspires Tom Riddle to do a trial run while creating his Dark Lord persona for the Wizarding War.