History:
When the Sage of the six Paths fought the 10 tailed beast the beast used its most powerful techniques and the battle left the area saturated in the foul chakra of the 10 tails. Because the sage rested after the battle the chakra escaped and found a man (I forget the name) and burned the demon mark onto his hand (the mark used for initiation into the clan now) the rest of the chakra went to the moon, trying to return to the 10 tail's body and now resides on the dark side of the moon.
Abilities: Due to the foul chakra of the 10 tails effecting us, our chakra coils reject all outside forms of chakra. Ad a result we are incredibly hard for med-nin to heal, we can't use sage mode (and I believe Curse marks?) We are immune to genjutsu, Gentle fist and Kiyoshi techniques that insert chakra into us. We summon Keigoku (chakra of the 10 tails) with portals of varying size that teleport the chakra to us from the moon. When Keigoku touches non-Nekura ninja it causes intense physical pain in their brains.
Edit: enough exposure to Keigoku can leave them in a vegatative state.
I think I covered everything.