Warforged

There is a ring that holds the wearer's soul inside of it at the moment of their death. Traditionally their soul remains in the ring until they choose to move on. But the ring can also be implanted, much like a heart, in a body made of wood sourced from druid trees, armored with the finest metals, to create an immortal warrior. The Warforged were created as a means of undercutting the Devil Legion. Someone whose soul was promised to the nine hells could avoid their fate with eternity in service to prepare for the Last War.
Today you can volunteer to become a warforged or you can be sentenced to such a fate much like The Night's Watch in GoT.