Crevasians are conservative and withdrawn people living close to the mountains. There is remarkably little interesting about them, and they tend to stay together anyway. They do not travel, so few care. Oh, and just one thing: they are NOT humans.
Crevasians most probably were humans long ago, but whether it is a matter of some random mutation, or a magical incident, is not sure. Differences are several, but few were ever noticed by outsiders, except for that thing with their veins and skin.
Their skin is indeed thinner than most races have, and while it protects against dangers of outside like it should, it is considerably weaker against inside forces. If a Creavasian is in an intense emotional state, or under hard physical stress (meaning faster heart-beat), his/her veins can thus burst open through the skin, and blood freely flows out.
This is of course a major disadvantage. While adopting a careful lifestyle, Crevasians have created a number of mental exercises and techniques, for control of both emotions and body. Many are now able to close such veins quite fast with strong concentration.
Another technique is said to come from a pair of lovers, and has deeply changed the little enclave. Forbidden for a few centuries, it was used again and again, because of danger or curiosity. It is widely used now. If one is wounded or sick, they could open a vein, and a close person would open theirs, and with the flow bringing new blood and strenghtening the immune system, they could be cured of their ailment. Of course, some types of blood can react violently (even fatally) with other types. Whether they are able to discern the difference, or could have now all the same blood group, I leave to your choice. (They might very well be uncompatible with ordinary humans now, though.)
Anyway, controlling the very blood flow has become a serious need. Training combined with genes favouring the proper enzymes, created a different race. While still staying away from physical harm, very few suffer of diseases that can the immune system of anyone of them cure. Stimulating the proper organs, some can release the right hormones, vitamins and other important substances, on demand, and be so even more effective healers.
The society of Crevasians was long aware of its hidden weakness, and many indeed see them as weaklings. But recently, another view is coming into favour: they are in fact the more evolved, “higher” life form. That useless violence aside, they are much more fit for life, and who has vaccination in these primitive times anyway? Another strong side is their strong communal feeling, which _literally_ flows in their blood…