I've played a lot of Roguelike games, at this point I've been playing various forms of The Binding of Isaac for literally half my life. Not to imply that I'm an expert, or even particularly good, but you could say I'm familiar with the genre. So, all this is to say, it would behoove you to believe me when I tell you: something Really cool just happened.
First, some context. The Roguelike genre is one with a high amount of plasticity, pretty much any game with a permadeath system can be considered a "roguelike" if you're willing to be pedantic. That being said: the structure of most, capital R, "Roguelikes" is a series of increasingly difficult levels culminating in a "final" boss. Every level typically ends with, or at least contains, a boss fight; of course this doesn't go for every game in the genre but this is the general structure, seen in games like Enter the Gungeon, Nuclear Throne, and the Risk of Rain games. The main gameplay loop is somewhat different from other games because when you lose, you start from the beginning, sometimes you get some upgrades so your next "run" easier The problem is that these bosses often become some of the most boring parts of your run, especially during the early game. Patterns are memorized after a handful of runs and then you spend the rest of your time with the game dunking on that boss. Not to say that there aren't tough bosses in Roguelikes, anyone even passingly familiar with the genre will tell you that some of these games will kick your ass up and down the street but the point is that it's the same boss doing the kicking. It's just after your 300th run, the first area boss becomes nothing more than a formality, you have learned all of their moves and they couldn't hope to damage you. The understandable patterns that make bossfights winable in the first place sabotages their ability to challange players 200 hours down the line. This seems like an intrinsic element of boss design, the bosses have to be consistant so the fight is fair but this brings me to Hades, the new Roguelike from the creators of Bastion, and how they manage to change up bossfights consistantly through the game, even after 100 hours while keepings things fair and understandable.
Before getting into the meat and potatoes I need to take a moment to praise the writers at Supergiant. Almost every boss encounter I have seen has started with a new bit of (mostly) fully voice acted dialogue that is not only unfailingly charming but efficient characterization in where most games would just have a splash screen and have the fight begin. Let's look at the first world as an example of this, when first attempting to escape hell, your character: Zagreus (prince of the underworld) runs into one of the 3 legendary furies of the underworld, this is the boss of world 1 as well as Zag's ex. As you progress through the game you meet her sisters, the remaining furies, as they take turns being the acting as end boss. You get to know all of the furies and when you're finally back on speaking terms with your ex you can come to learn how they feel about each other, both as furies and sisters. As you get into the game and get your first dozen or so runs under your belt the boss(es) of the first area will begin to comment on the repetition and, to my delight, about 40 runs in, your player character gives a nickname to the Hydra living under a river of lava, forever changing how the characters and in-game menus refer the to the creature. This level of dynamic, character driven change is not only a major step forward in and of itself but is easily one of the largest reasons I kept coming back to the game, I'm so charmed by all of these little guys and I can't wait to spend more time in hell with them.
If it was just good characterization and quips, it would be interesting but similar to other a lot of other games once you actually start fighting properly. This being me to the real meat and potatoes of Hades, the pacts of pain. Once you prove to the game that you are able to beat it a few times it, Hades (the god, not the game) will attempt to further increase the difficulty of escaping hell with a contract that places more difficulties upon the player, with the promise of greater rewards. One of these conditions is called "Extreme Measures," which "arms the Underworld Bosses with new techniques." Activating this particular clause will change totally change the boss fights for your run. Now, instead of coming at you one at a time the furies attack together, their simple attack patterns coming together to make a fight that is more difficult and exciting than the sum of its parts. Same goes with the second boss of the game, the bone hydra,who's arena changes so drastically that you have to completely rethink your tactics. These things do not make the bosses infinitely complex, they still have their limits and all this really does is prolong the time from first encountering a boss to being able to steamroll it every time but it is a glimpse of things to come in the future.
Hades manages to implement the kind of modular design that has been a staple of the Rougelike genre for years into its boss fights. Both through randomization and player controlled difficulty options. In essence this is one step closer to a "pure" Roguelike, one which the design principals and sensibilities apply to every aspect of the game, creating a uniquely focused experience. We're most likely not going to see games all of a sudden adopting these ideas, but now that we are living in a post-Hades world, boss fights will never be the same.