Bloodborne has three known covenants - the Hunter of Hunters, the Cainhurst Vilebloods, and the Executioners.
Bloodborne guide - covenants
Bloodborne players are yet to unlock all the secrets of covenants, but several have been identified and the basics of joining and effects are pretty well sorted out.
You "join" a covenant by equipping its Rune, so the process of becoming a member involves completing a sidequest to earn this reward.
You can can switch between covenants at any time simply by returning to Hunter's Dream and swapping our your covenant rune. At present, there is no known downside to doing so.
Here’s what we know about the first of these optional allegiances.
Hunter of Hunters
The first covenant you can start working on joining is the Hunter of Hunters. Its avowed intent is to hunt down those Hunters who have become "blood-addled" - started turning on innocents. Members will sometimes be called into the worlds of other players who have committed foul deeds, and challenges with punishing them.
To join the Hunter of Hunters covenant, you need to complete Eileen the Crows' side quest.
If you have gained access to Forsaken Cainhurst Castle, you can join the Cainhurst Vilebloods. These opportunistic Hunters advance through the ranks by collecting an item called Blood Dregs, harvested from other Hunters - both NPCs and other players.
To join the PvP and invasion focused Cainhurst Vilebloods, visit Annalise, Queen of the Vilebloods.
Like the Hunter of Hunters covenants, the Executioners punish other players. In this case, though, their goal is very clear cut: the Vilebloods must die.
Joining the Executioners is a multi-step quest involving Alfred.
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