The Twin Princes - Dark Souls 3 Boss
I really enjoyed this fight as well, but I didn't really get the "primary antagonist" feel from them or from any part of the game. If they were the reason the Thrones went empty... I feel as though there should have been more evidence in the game. That said, there COULD have been a lot of things I've missed... I've only gone through the game once.
I manage to Solo the Twin Prices in Dark Souls 3, but was unable to beat them on my first try.