Lethe is one of the Primordial Gods that Viktor Welsh initially theorised but then started to collect her physical and psychological pieces to try and prevent the apocalypse.
In contrast to her sisters, Lethe has eight arms instead of two or six. Viktor Welsh has theorised that each of the other six sisters provided her with an arm so they could also be seen to have a hand in the end of all things.
Upon realising what her nature is and what she would be expected to do - i.e. destroy all worlds during an event called the Big Crunch - she decided to destroy herself in an attempt to avoid the inevitable. Her mind and body fractured into pieces and hit the world like a meteor storm. One of her arms landed in the area where Rhodskirke would be later built.
Her reach is still felt - whether it is the Forgotten Priest languishing in the Aetherial Plane or a small group of devout followers who are uncertain of the Synergy Group's methods and reasons.