Board Thread:Manga/@comment-5774380-20161007195718/@comment-27321453-20161011065947

Tausendberg wrote:

Maybe the dumb dumbs are actually the result of the Marley's experiments into trying to make more shifters. Yes that's a possibility and when they kept failing, they opted to use it on any Eldian revolutionary from the ghettos and send them to the island sieging their own people.

So essentially they squeezed a benefit out of their failed experiments.