Assassin's Creed III (2012)
Assassin's Creed III is the forgotten child of the series. Coming in between the lukewarm Revelations and the comeback kid Black Flag, people often forget that III was another major shit for the franchise.
While the three games in the Ezio trilogy felt largely familiar, III introduced a new engine, a single, large open world, hunting, naval combat, and much more. It certainly addresses Revelations' criticisms of familiarity with a much more ambitious game. The missions were still scripted, though, but it was clear the series was gradually moving towards a more open-ended, open world structure.
The narrative is also excellent - the twist after the first section, switching characters with Connor's coming-of-age tale, the American Revolution, the father-son drama all mark a high point for the series.
Unfortunately, III is also burdened with concluding the modern day story of Desmond. It's often too much, and detracts from the more interesting American Revolution story. While the many new mechanics definitely helped in refreshing the series, it never really came together as a coherent whole. Like Revelations, there was a lingering feeling that the yearly cadence was starting to hurt the franchise by rushing games.
Still, Assassin's Creed III doesn't get the credit it deserves - it's definitely one of the better games in the series.
Assassin's Creed IV: Black Flag (2013)
Somewhere between Brotherhood and Revelations, Ubisoft realised the 2 year development schedule was unsustainable, and split the Assassin's Creed development into 3 teams. This game each team 3 years instead of 2 years, but the franchise could continue to maintain its yearly cadence. This was also the first game to have significant assistance from many of Ubisoft's satellite studios around the world, in a Rockstar-eque move.
The first fruits of this change appeared brightly with Black Flag. Though Assassin's Creed III is widely thought of as the end of an era, Black Flag actually continues on from III's engine and mechanics. The historical story is also more of a prequel to III, while the modern story gets rebooted.
While III had its share of open world chops, Black Flag was the series' first foray into a massive open world not unlike Skyrim. Naval mechanics were expanded from III to play a larger role in this pirate themed adventure. Although stealth still played its part, dynamic free-form combat was much more prominent. So too was the mission design - overall, this was the game with which Assassin's Creed started to resemble a modern open world game. Everything that III promised, Black Flag finally delivered in spades.
Black Flag was a significant milestone for the series. The concerns posed by Revelations and III were forgotten, and the franchise had found a new identity. Strangely enough, it was rather a one-off, with the series going in a different direction the next year, with Unity.
Assassin's Creed Rogue & friends
Well, not quite. Rather oddly, Black Flag's sequel was Rogue, a one-off spinoff developed by Ubisoft Sofia. It released alongside the flagship next-gen instalment, Unity, and was expectedly forgotten.
Why does this game exist? Well, Unity was a next-gen game, and Ubisoft wanted something playable on the older consoles and PCs. It also serves a narrative purpose, concluding the Conway story. As a result, III, IV: Black Flag and Rogue form a trilogy by itself. At the same time, it also links Black Flag to Unity.
Rogue definitely has some really nice narrative moments, and a worthy conclusion. However, the gameplay and missions always felt like a B-team version of Black Flag lacking in innovation.
There were also three Chronicles games, released as 2D platformers. I've only played China, which was rather mediocre, and didn't bother playing the other two.
Rogue and Chronicles would prove to be a dead end, with no further such spin-offs developed in the franchise. After Unity, the series would consolidate itself once again into its current model - but more about that in the next part.