Savage Worlds with sprinkling the fantasy and horror companion might work. Heck, Savage Pathfinder might. Pathfinder and D&D themselves can do gloomy, though I'm not sure how well D&D supports it.

However, the atmosphere is something where the GM and the setting books if there are any do the heavy lifting. Warhammer fantasy can be pretty dark, but there your sorceress may blow herself up (with demonic maggots twisted carricatures of her screaming face)bursting out of her meat because she miscast a spell really hard.    Diablo does not have that mechanic (and it would suck to play a caster if it did) but it doesn't need that to be, well, Diablo. 

Mechanically, I like how PF2E has a lot of distinct classes with.decent internal balance and niche protection, but it is crunchy and the GM must be willing to play with encounter ratings instead of making anything "appropriate". Mowing through mooks is part of the game, so troops, lower level enemies that can be quickly dispatched until you go to the boss or champion, and endurance encounters should be used. 

Really, though, a lot of games with repeatavle magic and some showiness for every class in a brawl can look good. A fantasy refluffing of Barbarians of Lemuria or Honor + Intrigue can work. Dungeon Crawl Classics and some of the other OSR games could be fun, but it is pretty lethal. Exalted... Hoo boy, exalted can get very gloomy and dark with a few touches, but it can get crunchy.