If you do eventually fall, it will be as the Master fell, knowing the evil you do, welcoming it. The abuse of power will push you in that direction. On the other hand, its mere use may drive you the other way, toward our god. That is what it means to be a Shanir, to walk the knife's edge.
– Immalai, Dark of the Moon, "Chapter 1: Fire and Ice"
A Shanir is a Kencyr with powers and attributes drawn from the Three-Faced God. It is an inheritable and genetic characteristic that can be bred for, and is carried in Highborn bloodlines to varying degrees. Kendar can be Shanir too, but only if they carry Highborn blood.
Before the Fall, Shanir were viewed favorably, and Kencyr inbreed to produce powerful Shanir. Then came Fall. Gerridon and Jamethiel Dream-weaver were the product of inbreeding, and themselves bred together. Thought they had no child, they metaphorically bore the Fall through their Shanir power. Shanir were scapegoated, and people no longer trusted Shanir to use their powers honorably.
Some of these are more rare than others. Many of these traits are regarded with deep suspicion by the Highborn Kencyr after the Fall, when the Highlord, Gerridon, and many of his followers, mostly strong Shanir, betrayed the Kencyrath to Perimal Darkling.
In addition, the strongest Shanir are prophesied to become the Tyr-ridan, avatars of their triune God. Since the Fall, the Kencyr view their God with little more trust than Perimal, so the strong Shanirs are doubly distrusted. However, the Kendar, and particular the mostly Kendar officers and soldiers, have a more pragmatic attitude to these talents; seeking to make use of them as any other skill or weapon.
↑God Stalk, "Appendix III: The Kencyrath" — "Historians suggest, though, that long ago the Highborn may have been trying to breed Shanir, from whose ranks the Tyr-ridan or chosen three will come to lead the Kencyrath in final battle against Perimal Darkling. The way to get the most powerful Shanir, however, is to inbreed, hence the custom of mating twins. But since the Fall, Shanir have become less popular and cross-breeding between houses has become more and more the rule."
↑Seeker's Mask, "Part I", V — "Of course the shadows would be defeated, as soon as the three aspects of their god deigned to manifest themselves in the three Shanir known collectively as the Tyr-ridan. Earlier matriarchs had tried to speed that day by mating together potent Shanir. Whether they should have done so, especially within the bounds of blood-kinship, was still hotly debated. One such match had produced those dire twins, Gerridon and Jamethiel Dream-weaver, who in turn had been bred together with no issue except, perhaps, the Fall itself. Nonetheless, some matriarchs believed that in general their predecessors had in general the right idea."
↑God Stalk, Book II: Crown of Nights, "Chapter 7: The Feast of Fools" — " 'I suppose,' she said slowly, 'that it's partly because we no longer trust anyone to use such gifts properly. Of course, the ability to touch minds with an animal isn't all that threatening, but what about those who can weave dreams or whose blood, once tasted, binds a man body and soul? Our history is full of strange people, Dally, with stranger powers. One of the strangest is the Master.' "
↑Dark of the Moon, "Chapter 12: Night Pieces" — " 'Then came the Fall. As far as I know, the Master wasn't a blood-binder; but he did abuse what Shanir power he had so spectacularly that afterward we of the Old Blood were made the scapegoats for all of our people's sins. But I hardly have to tell you that. As you well know, all the Shanir talents came under suspicion, even the beneficial ones.' "