Beta Testing the Apocalypse

MC accidentally gets shot in the head and ends up meeting a being known as a Transdimensional Fairy. After forging an agreement, he is sent to a localized world in Purgatoria to become a Beta Tester for humanity's upcoming apocalypse. His objective is to conquer a Tower of unknown height and to aid him in his journey, he ends up summoning various characters from his memories, the first being Mordred Pendragon from the Fate Series.