Note: These might be unofficial and are not yet fully confirmed. Credited to people who uploaded images/videos/information.
How to get
[Shadow Game] At the end of their respective turn, each player will lose LP equal to "100" times the number of cards in their own graveyard.
[Right Back at You] Can be used in the Main Phase of your next turn following the turn in which you took battle damage. Select 1 face-up monster on your field. This turn, the selected monster's ATK increases by half of the battle damage received in the turn you last took battle damage. This skill can only be used once per turn.
[More skills are expected.] Report Skill name and details by submitting comments or screenshot at the bottom of the page.
How to unlock
There has been no official announcement about how to unlock Yami Marik, but, at Jump Festa 2016 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.
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.
When you achieve Stage 50, you're told that there will be more stages added at a later date, and that more duelists will show up to challenge you eventually (Showing off pictures of Pegasus and Yami Marik). That being said, I think that points to them not being available in game just yet but eventually will join the ranks through character events, I would presume.