Damon and Elena do not both become human in The Vampire Diaries. Damon remains a vampire throughout the series. Elena Gilbert, however, is tied to Bonnie Bennett's life through a spell cast by Bonnie's mother. When Bonnie dies at the end of Season 5, Elena falls into a magical sleep. She awakens in the series finale (Season 8, Episode 16, "I Was Feeling Epic") only because Bonnie sacrifices her own immortal life, and Bonnie dies permanently so Elena can wake up. At this point, Elena is still a vampire—she doesn't become human. The show ends with Elena ultimately becoming human much later in life after decades pass, but this occurs in a time-jump epilogue scene set in the far future, long after the main events of the series conclude. The core relationship between Damon and Elena as vampires is central to their romantic arc throughout most of the series.