Everything? O_O Well, it's been a few years, so I'll do what I can remember...:
Back during the time of the Sage of Six Paths, after he sent the body of the Juubi into space, trapping it in the moon, he rested. When the Sage rested, chakra from the foul beast left behind on earth sought out our ancestor, who was a bitter person... I can't remember his first name. O_O But his last name was Nekura....
All members of the Nekura clan have the Mark of the Demon somewhere on their body; it's common to have it on one's hand, like the first Nekura, but some people have it elsewhere, like myself where it's over my right eye. The Mark of the Demon is the "bridge" which connects the Nekura clansmen and the Juubi in the moon. It's because of the mark that the Nekura can do what we do...
Through the Mark of the Demon, the Nekura are able to pull the foul chakra of the Juubi from the moon where its body is sealed and into this world through a small portal we can generate anywhere in our general area. The chakra of the Juubi is foul and corrupt, making it so when the chakra touches another sentient being, their mind becomes overwhelmed by the chakra and they will be unable to think straight; Nekura techniques are mind-crushing jutsu. We call this chakra of the Juubi, Keigoku. The Keigoku is an intangible substance, allowing it to travel through objects of certain size and density, depending on the power of the user.
Because of the Mark of the Demon and the connection to the Juubi, Nekura clansmen have a few innate abilities, which I personally have been enjoying on my travels. For one, our body rejects foreign chakra because of the Mark, making it so that we are unaffected by genjutsu, Jyuuken, other clansmen's Keigoku techniques, and other harmful techniques which inject chakra into our bodies. That said, we are also immune to Curse Marks, Sage Mode, most Medical Ninjutsu, and a few other helpful techniques. In addition, our Keigoku jutsu make summoning animals turn against their summoner, as well as the fact that when a Jinchuriki taps into the chakra of their Bijuu, they will not fight us as the Bijuu sees us as the Juubi itself.
An ability of higher-up Nekura clansmen is the Drain. Uhh... Personally, I know very little about the Drain; it allows the user to take chakra from victims of their Keigoku jutsu, and it only available to clansmen well-versed in the ways of Keigoku, which does not include myself... Black Knight, maybe?
Anyhow, the Nekura clansmen, the ones born to the clan, are born to this country, the demon country Minas Morgul. One does not have to be born into the Nekura clan to use Keigoku, but they will have to undergo a burning sensation as they gain the Mark of the Demon.
Mystogan stopped to think for a moment.
Other than the fact that a higher-up member of the Nekura clan can use the same jutsu as a lower-ranked member with more power, for example the Clan Council's jutsu are twice as powerful as a seekers, even if they use the same technique, I can't think of anything else...