There has been no official announcement about how to unlock Yami Bakura. However, at Jump Festa 2017 that was held on December 16th-17th in Tokyo, JAPAN, visitors can play a duel against Yami Bakura, Pegasus, and Yami Marik visiting the Duel Links booth.
Yami Bakura shows big surprise on his face when he loses a duel...
Description in the game
When Yugi's classmate Bakura was possessed by the evil spirit inside the Millennium Ring, he transformed into Yami Bakura. He is merciless and will stop at nothing to achieve his goal. Yami Bakura's Occult Deck focuses on Fiend and Zombie-Type cards.