The Dark Elves are a race of people featured in Tenchi Muyo! War on Geminar. Their home country is the Shurifon Kingdom. It is revealed that they are not a native race of the planet; rather, they were summoned long ago from another world by the Senshi Clivilization in the hope that they would be better suited at piloting Seikishin.
Various elf species are featured in the Renza arc of the Tenchi Muyo! GXP novels. They inhabit magical planets in that area and are probably the source of the summoned elves in the Geminar dimension.
Dark Elves suffer a daily "weak period", when they revert to a weak-willed and very timid personality and are generally unable to function normally. This is apparently a side effect of their race being from a different world/dimension.
Dark Elves can also project a protective "Dark Elf Field", which can repel and disable an enemy Seikijin or other craft. This seems to be mainly a defensive measure and drains the users energy.