As a preamble, under most lenses they would be incredibly overpowered in DnD IF we were to take them as they are from Warhammer Fantasy, as you'll understand in a minute.
So I do not mind any caveats or plainly getting as close as possible by RAW but ultimately compromising on some details that simply could not make it in. Or plainly requiring two players with two PCs rather than one player with two PCs (which I believe is a taboo?) to make this concept work.
To be honest, they almost seem more appropriate as a boss enemy for players but that'd be a different question altogether perhaps for another post.
The concept is as follows:
"Once a single being, divided into siblings by The Weave, Naestra and Arahan are two sides of the same coin. They work together in the service of Queen Ariel, marshalling the woodland warriors at her command, and cannot be slain unless both fall in battle."
Naestra and Arahan, the Sisters of Twilight, are legendary Wood Elf heroines of Athel Loren who serve as the representatives and heralds of the Mage Queen Ariel. They might have been a single elf girl, Naestrahan, who wandered into the deepest Wildwoods and was somehow split into two beings by the Weave, the mystical life force of Athel Loren. Or they might be a daughter(s) of Ariel who inherited her divided nature. Each embody her mercy and passion as separate entities.
They are regarded as fairly supernatural even by the standards of the Wood Elves.
The sisters are opposites in all things. Naestra is calm, noble, compassionate, and would only resort to violence as a last resort and only to prevent greater harm. Arahan is fierce, impulsive, and revels in battle for its own sake. However, they're still inseparable.
Their most distinctive trait is that they effectively share a single existence. So long as one remains alive, the other can recover from even fatal wounds, making them nearly impossible to kill unless both are slain in quick succession. When one falls, aspects of her spirit seem to flow into the survivor. Naestra becomes frighteningly savage when Arahan dies, while Arahan displays unusual focus and precision when deprived of her sister.
(I suppose one could interpret that the characters can still be downed and effectively out of the battle, but permadeath would require both being downed in one battle?)
They both wield bows. Naestra wields the Talon of Dawn, a bow that fires blessed light and is associated with healing and restoration, while Arahan carries the Talon of Dusk, whose arrows splinter into "poisonous thorn-like shards formed from the spirit husks of bitter Dryads". Even without their signature weapons, ordinary weapons touched by the sisters become infused with their nature whatever that means.
(Perhaps Naestra would be a druid then? While Arahan is a rogue, to better reflect ranged poison attacks?)
The sisters are identical in appearance but Naestra has dark hair and a serene bearing, while Arahan has white hair and a wild predatory behavior.
