Additionally, it seems like the mention of the Varangian Guard might have confused things. That was more or less just the name given by the Eastern Roman Empire to the politically-independent mercenaries serving the Emperor directly. They happened to, initially, be derived mostly from Scandinavia--thus Vikings. With Viking dark elves and a human empire derived from the ERE, keeping that sort of dynamic would mean getting the guard of dark elves from wanderlust, desire for riches, or whatever, and having them serve as politically neutral soldiers--possibly moreso, due to lifespan differences between humans and elves. Not a separate subspecies.