Where can I find info on World Axis (4e) Sorrowsworn and Nightwalkers?

To the best of my knowledge, in 4th edition, Sorrowsworn were defined as powerful psychopomps who maintain the preservation of the cycle of death and rebirth, and/or as Exarches of the Raven Queen, and Nightwalkers were powerful "shadowy" undead (which I think wasn’t that big a departure from their previous lore) but I cannot recall what books specifically talk about these entities or what they specifically say.