The Magus is stated to be appearing in The Avengers: Infinity War.
The info comes from the Action Auditions casting call website that states the Magus will have a major role along with Thanos.
Here is the blurb:
Auditions are now getting underway for the Marvel Studios feature film ‘Avengers: Infinity War – Part I’ lead and supporting roles. The auditions for new lead and supporting roles will be taking place in Los Angeles, CA. The film is the first in a two-part sequel in which Thanos plays a major role, along with the Magus. Adam Warlock’s bad side, the Magus, separates from his body and assembles the Infinity Gauntlet. Magus creates doppelgangers of Earth’s heroes as part of his plan to recreate a universe of evil. “Avengers: Infinity War – Part I” will be shot entirely on the IMAX/ARRI 2D digital camera.
Obviously, if this info is legit, the inclusion of the Magus means Adam Warlock will get introduced at some point (Magus is the future version of Adam Warlock), with many fans online speculating Adam Warlock to be introduced in Guardians of the Galaxy 2.
Regarding the description of the Magus, the info may not specifically pertain to the movie as it's basically a description of the Infinity War Marvel comic; however, of course, anything is possible.
Long story short, in the Infinity comics, Thanos assists the Earth heroes and Adam Warlock in battling the Magus, who has aquired not only five extra-dimensional cosmic cubes, but eventually the Infinity Gauntlet as well.
The Avengers Infinity War Part 1 is due out May 4, 2018, with Part 2 on May 3, 2019.
Guardians of the Galaxy 2 hits May 5, 2017.