User blog comment:Master Sima Yi/Assassinews 05-08-'14 – Assassin's Creed: Rogue announced/@comment-4254431-20141119234251/@comment-25633871-20141120030250

It's one of the rewards for completing the Panthéon Nomad missions. They're high-level missions, I think they need level 17 or higher assassins. Hope that helps you.