Thanos

While he's won't be the only villain featured in Avengers: Infinity War, Thanos will certainly be the main antagonist of the film. We've officially known about the character's presence in the universe since the mid-credits sequence of The Avengers, which revealed him as having a desire to conquer Earth, and we got to know him a bit better in Guardians of the Galaxy through his interactions with Ronan The Accuser. The last time we saw him was in Avengers: Age of Ultron, when he finally decided to hunt for the Infinity Stones himself, though we did see what looked like his ship in the mid-credits sequence of Thor: Ragnarok. In the third Avengers adventure, we will see Thanos finally pursuing all of the Stones in the hopes of assembling them in the Infinity Gauntlet, which will see him effectively gain the powers of a god and gain control over all time and space. Naturally, it will be up to the Marvel Cinematic Universe's greatest heroes to stop him and his minions, known as the Black Order.