There has been no official announcement about how to unlock Yami Marik, but, at Jump Festa 2017 that was held on December 16th-17th in Tokyo, JAPAN, visitors can play a duel against Yami Marik, Pegasus, and Yami Bakura visiting the Duel Links booth.
See what happen when Yami Marik lose a duel against you...
Description in the game
Yami Marik is Marik Ishtar's dark side created from the suffering he endures as a Grevekeeper. Cruel and vicious, he enjoys watching his opponents suffer. His appetite for destruction is incomprehensible. His "Lava Golem" can be summoned to the opponent's side of the field where it slowly puts the stranglehold on his foe.