Assassin's Creed IV: Black Flag is basically a new-gen, more murderous, stealthy, parkour-packed version of Sid Meier's Pirates! Plus, it has unlockable sea shanties. If you use the 5% off Facebook voucher at CDKeys, you can get Black Flag on Xbox One for just £7.59.
NB. Do bear in mind that this is a digital download code. Thanks Mark!