User blog comment:Xilinoc/Ch. 584 - THE HEADLESS STAR 3/@comment-5551986-20140611182431/@comment-5693502-20140612154004

I've already had a debate elsewhere on why Hollows are still technically souls but, besides that point, that idea on sealing makes a lot of sense. I've always thought that Yhwach is something other than the races we know of and him being something similar to the Spirit King sounds quite 'right'.

Wild theory alert: What if Yhwach is the "husk" of the Spirit King ? As in the Spirit King shed/split from Yhwach (in some way) and this is the reason he was initially rendered a sensory deprived vegetable. The souls he absorbed fill his husk-like being (with power ?) enabling him to temporarily function properly. His actions could then stem from a subconscious wanting to return to being "whole" again, or at least be "filled".