In Garth Nix' Sabriel Trilogy, Astarael is the name of a bell used in Necromancy. As the story develops, it is revealed that Astarael is not only a bell (like the other bells), but is an entity, somewhat akin to a Sefira, that played a roll in the world's creation.
Their are seven bells that are used by evil necromancers and the good Abhorsens, with Astrarael being the largest and most powerful, having the power to throw anyone who hears it into "death", physically and metaphysically. The bell's title is "The Sorrowful", or "the Weeper".
Further on in the trilogy, there are more insights into the relationship between the bells, the "Charter", and the original entities that formed the world. The Charter is a magical system of balances that provides order and structure to the cosmology of the world. It was originally put together by seven powerful spirits. Of these spirits, five went into the Charter, while two supported it but didn't lose their individual existences. Astarael was one of the two.
In a scene in the third book, two of the characters who have grown up with the magical and sociological support of the Charter their entire life confront a remanent of Astarael, in the form of a spectral, ectoplasmic woman. She causes the charter to fail around them, something that they can not even comprehend. It is only through the efforts of the other free spirit that they manage to escape.
The idea of Astarael being both a support of the charter, and outside of it, seems to relate to the idea of "sorrow". The Charter provides a framework for people to feel continuance and connection with the universe. There are some feelings of sorrow, Garth Nix seems to be suggesting, that can not be totally integrated into the world, because they speak of an ultimate end, and an ultimate parting, beyond the idea of the neverending continuance of the world around us. However, these feelings, even though they speak of an end, are still neccesary for the Charter, and by extension, are neccesary for a sense of contiuance to be meaningful.