User blog comment:Xilinoc/Ch. 579 - THE UNDEAD 6/@comment-24913480-20140507201544/@comment-1268838-20140507222852

Well, the power itself come from the "Schrift" that Yhwach engraves on their soul with his own. Presumably, that entire package returns to him once they die, but at that point I think it all just merges back into his own soul. So no more letter, no more power; otherwise, by the end he would have the entire alphabet engraved on his soul. Which is a possibility, I suppose.