It could be cultivation working through the Nalthis peoples, but what i found interesting was I believe what Vasher implies in rhythm of war is that the returned are returned to their own body out of convenience, not necessity, so in theory Nightblood COULD be a returned himself, and with the obvious use of them copying shardblades, it could be Endowments god metal as well.
But this is just a fever dream I had last night, If I messed up my details or if this has already been debunked please LMK.