Mashou no Nie 3 ~Hakudaku no Umi ni Shizumu Injoku no Reiki~

Long ago, the infamous pirate Van Clad ruled the seas unchallenged—that is, until his cunning navigator orchestrated a mutiny and slayed him during a treasure expedition. However, before drawing his last breath, he swore to one day return and claim his vengeance upon the descendants of his betrayers.

Three hundred years later, the young naval officer Klaus Ainhart stumbles upon Van Clad's final resting place. While exploring the deceased pirate's remains, Klaus is suddenly possessed by Van Clad's malevolent spirit. Now in possession of the officer's memories, the newly reborn pirate learns that his host is actually a celebrated military figure from the Kingdom of Austria, founded by none other than the traitorous navigator who killed him centuries ago.

Under the guise of Klaus, Van Clad carries out a sinister plan to defile the legacy of the man he hates the most—by enslaving the women of Austria and turning the kingdom into a pirate's lair.

[From Myanimelist]