The curse of Green Knights is that they cannot die without an heir to assume their place. This goes as far as denying them death of age, and any sentient or thinking creature which slays the Green Knight automatically has the Green Knight's power, status, and contract passed on to the victorious survivor. In this way there is no end to a green knight lineage. Not even suicide is an escape from the responsibilities of a Green Knight.
An honor to become one, they are an organization of Feywilds Knights in service to an Archfey. These knights are dispatched by the Seelie Courts to handle threats to the realms. As part of the ritual to become a Green Knight, a creature that is not an Eladrin Elf is granted their form along with powers associated. These are the right hands of Queen Titania