With all six Infinity Stones, he will be able to use them to 'inflict his twisted will on reality'. In Infinity War, Thanos has 'emerged from the cosmic shadows' and is ready to take what he believes is rightfully his: the Infinity Stones.
The film's synopsis all has to do with Thanos, who is the main villain of this movie as well as the following Avengers film. Understandably so, since it will have every Marvel hero to date in the film and is supposed to have intense action sequences that are unrivaled. Avengers: Infinity War is the most anticipated superhero film, and film in general, of the year.
