I'll refluff some of them, sure. The Blood War doesn't really exist in its typical D&D form, no. But there are still factions of fiends in Niflheim. Sometimes they'll work together, and sometimes they'll betray each other, or go into outright war. It's a lot more "tribal" than typical D&D fiends, but a Pit Fiend is more likely to trust and side with a Horned Devil than a Balor.