To access Monad, you must first unlock "Heaven's Door" at the highest level of Tartarus (floor 254). Accomplish this by first accepting and complete Request #53 for Elizabeth, "Retrieve 1 Bloody Button." This requests requires you to lure out and defeat the Reaper on any floor in the dungeon. Then return to Elizabeth when you are finished to receive your reward, and open the way to Monad! This also unlocks Requests #55 and #72.