Diablo.
I ain't even kidding.
I know it'd be somewhat superficially similar to Castlevania, but hear me out for a bit: don't focus on the action of the game and don't even get really deep into the Sin War lore. Start by making it about Diablo's slow corruption of the town of Tristram. The first game's gothic horror vibe lends itself perfectly to a brooding tale of a slow descent into madness with some big religious undertones, set in a very Dark Age fantasy location. Focus on the illness/corruption of poor Prince Albrecht, focus on King Leoric getting more and more desperate and unhinged as time goes by, changing from a decent (if slightly arrogant) ruler to a full-blown maniac, focus on Lachdanan and how he went from unflinchingly defending his King to slowly realizing he has to do what he dreads most and becomes a reluctant, yet honorable regicide, focus on Archbishop Lazarus pulling the strings behind all that palace intrigue and corruption. Treat the Sin War stuff as "legends" in a storybook prologue, so don't make it the meat and potatoes of the series - just background stuff to serve as general context.
And after that first batch of episodes (let say, 4 out of an 8-episode season) start to focus on the heroes that end up fighting Diablo. You can tweak the story slightly and make it so all of them reach Tristram during its final days, before Lazarus leads the townfolk into the Butcher's lair under the Cathedral (instead of each of them going to Tristram after that happened) so the transition between "halves" isn't that abrupt. After that it's all about those heroes and maybe include references to some of the original game's sidequests, make Cain and Adria advisors to the heroes and then start to slightly introduce Sin War stuff into the foreground (mainly about Diablo's overall plot, the Horadrim's mission and who is Deckard Cain) just so it all starts to "make sense".
Of course, the season would end just as the first game: a bittersweet, ominous tone. Diablo is apparently defeated, just barely contained within the Soulstone, and the main hero piercing his head with it believing himself to be mentally/spiritually strong enough to withstand the Lord of Terror's corruption. A small glimmer of hope for Tristram that we all would know would be shortly quashed beyond recognition.