It is a title for shinobi who fight yōma on the front line. Good, Evil, and even Renegade shinobi can obtain this title, as it requires nothing more than veritable invincibility. Becoming Kagura is the ultimate goal of all shinobi who are aware of the existence of yoma. It is said the purpose of the Shinobi Battle Royale was to cultivate Kagura that would be able to defeat yoma.
The title, according to a TS file in Burst Re:Newal, derives from True Kagura, the destroyer of Yomas. It is thought that the reason why Kaguras are so powerful is because they were designated bu Kagura herself as her vassals to fight Yomas.
They are because of that frequently affected to teams composed of Elites, and sent in mission to fight Yomas. But even if being Kagura make them invincible to Shinobi common folk, the hardness resulting of the trickery and cowardice of Yomas, and their huge power, still make the fact of being one not enough to counter them. And usually, it ends up with the death of the whole team.
Coupling that with the manicheism that split shinobies in "good" and "bad" and renegades in SK's universe, and one of the sides might think it would be a nice idea to ambush a Yoma-tracking expedition. That's how the Yoma incident that involves the death of Ryouki, and Miyabi's berserk happened. And the presence of two Kaguras, but opposed, doesn't seem to have changed anything.