Castlevania: Harmony of Despair
At it's core the Castlevania series is about heroes going into Castle Dracula, fighting monsters, dealing with traps and dangers, and overcoming the tough difficulty all for a chance to take out the Dark Prince, Dracula. Every game in the series (with few exceptions, since Dracula almost invariably shows up in all of them) can be boiled down to this formula, and it worked for nearly 30 years across the whole of the main series.
That's part of what makes Castlevania: Harmony of Despair (titled Akumajo Dracula: Harmony of Despair in Japan, with "Akumajo Dracula" of course translating as "Demon Castle Dracula") a particularly interesting victory lap for the series. By this time, Koji "IGA" Igarashi was already likely leaving Konami to work on his own projects -- he only released a couple of more, non-Castlevania games after Harmony of Despair; control of the main series then went to Mercury Steam and Kojima Productions for the Lords of Shadow series.
What Harmony of Dissonance does is look back at the highlights of IGA's tenure with the series, acting almost like a best of for his works and the Castlevania series as a whole. It has areas, characters, and moments that pull from (almost) everyone of of his 2D titles (with only the similarly titled Harmony of Dissonance, the weakest performer of the portfolio, missing out) all to make a massive, crossover title for the fans.
Although the game can be played single-player, the meat of the game in in multiplayer. In both modes players take control of any of a selection of characters from across the series (mostly the Metroidvania titles, but a couple of 8-bit characters were added later as DLC content) and guide them through six main stages (and another five DLC stages), fighting the hordes of demons, collecting a limited amount of weapons and items, and performing a marginal amount of leveling up, all to try and be strong enough to take on the bosses at the end of each mission.
Divided into 30 minute chunks, the stages provided large, expansive areas to explore while the challenge was intense enough (especially on Hard mode) to keep players interested for a while. It was a cooperative game, with the players working together to beat the stage within the 30 minute mark, and each player death pulls from the timer as they respawned, meaning the group was rewarded to not only working together but ensuring they all stayed alive. It was an intriguing concept and it worked well within the concept of Castlevania, with heroes all fighting the undead (instead of each other) to save the world.
Harmony of Despair wasn't a perfect game, with the core of the game being a very stripped-down experience that fans of the Metroidvania games may not have liked. But as a last hurrah, a statement from Igarashi saying, "this was Castlevania while I was in charge," (presumably with a mic drop afterwards), the game works. This was Castlevania of the IGA era and what came after was something drastically different indeed: the Castlevania: Lords of Shadow reboot.
It is worth noting that Harmony of Despair was something of a success. Although firm sales numbers for the digital-only game are hard to come by, the title did well enough to move off of just Microsoft's Xbox storefront, eventually coming to the Sony PlayStation and Windows storefronts as well. It received five main DLC packs, a number of character packs, and music DLC packs, too, providing a fair bit of content during its lifespan. And, even after the game ended, its concept has continued being used for the mobile games Konami has produced in its wake: Castlevania: Grimoire of Souls and Castlevania: Moonlight Rhapsody.