FElagi
A witch's life, before entering a coven, is often marked by solitude and hardship, as witchcraft is frowned upon and often hunted within the regular human society.
Hence the bonds between witches of a coven tend to be even stronger, almost as if to substitute for lack of acceptance they might expect from the outside. However, no bond is stronger than that between two Felagi.
When a new witch enters a coven, they get initiated and accepted by another witch that will mentor and foster them to be their best self. This is to strengthen the coven, its integrity but also to strengthen the young witch themselves. Learning the ways of the coven, discovering their expertise and powers, all these don't come naturally to most - they need guidance and mentorship and more importantly, they need trust and support.
Two witches are bonded for life as Felagi, they council each other and share a close connection. It may start out as a mentor-mentee relation but over the years, as the two witches grow more equal in their skill and experience, it simply becomes a deep-rooted friendship. Often their powers grow stronger if cast together, their energies so closely intertwined and fostered by each other.
Of course it happens in this dark and cruel world that one witch might lose their Felagi - to natural causes, to the mundane human population that seeks to destroy what they cannot understand, or even to a witch of another coven. When that happens, the surviving witch will feel that loss forevermore. It is like a soulmate being ripped away from you, leaving only a void that none will ever be able to fill. New connections might come and go but they can never replace what has been taken from them.
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A witch can usually has only one Felagi at a time, may it be as a mentor or as a mentee. It is custom that witches who have lost their Felagi are the ones who will take on a new witch joining the coven as their new Felagi, continuing the line of guidance and trust.