Since they describe it as a doom metal album of a game I'd suggest you go with doom and adjacent stuff such as sludge/post metal and such. These genres are also lower on vocals and better as background music than some others, imo.
Post metal-ish stuff: the ocean, isis, neurosis, cult of luna, russian circles, intronaut, pelican, dvne
Clear vocals doom either classic/epic stuff like black Sabbath (duh), candlemass, saint Vitus, reverend bizzare Or more "serious" stuff like isole, while heaven wept, warning - watching from a distance, thy listless heart, pallbearer, solitude aeturnus
And then there's funeral doom which is heavier and fewer vocals but there's growls. You've got Ahab, Bell witch, shape of despair, evoken, thergothon, doom:vs, funeral
There's also the more "stoner" side of doom like sleep, electric wizard, yob (amazing stuff), bongripper (amazing instrumental stuff despite the silly name), acid king, monolord
And other stuff that's good but just didn't fit there cause there's too many different styles: my dying bride, swallow the sun
Some of them have big enough catalogues that if you like their vibe, you can just stick to that and be fine. Like a playlist that's just Ahab or just bongripper and you're done. Ez pz.