All sorts of campaign settings can be invented for Dungeons & Dragons, but Dark Souls might offer a fairly compatible universe for 5e adaption.
Part of the brilliance and appeal of tabletop role-playing games like Dungeons & Dragons comes from player and Dungeon Master agency and the endless opportunities to improvise. There are plenty of official module books that will guide a D&D party through a campaign, but the leniency in game design lets DMs create their own settings – or even borrow ones from other sources. Particular video games, specifically Dark Souls, already have established universes that a D&D ruleset like 5e can be grafted onto.
Adopting an existing setting for a D&D campaign can help quicken the preparation process, but it’s still a lot of work, especially when the source material is steeped in obscure lore and signature game mechanics like Dark Souls. The fantasy settings of both D&D and Dark Souls make them fairly compatible, though, allowing DMs to lean one way or another to borrow items, spells, enemies, and dungeons. However, a Dark Souls D&D 5e campaign will have to make some significant changes to the 5e rules.