But yeah, it depends on how those powers are going to work. Even without the idea of an infinite-power-battery I'm interested in what happens when a Darkness user drains power from darkness.
For the purposes of the RP "draining" darkness would require there be a finite amount and you would leave something in its stead which one would probably assume is light (but that's not how that works). A dark Aedis who is drawing on shadows for example for power would eventually "destroy" the shadow. Destroying the shadow of an object will cause the destruction of the object. Destroying the shadow of a person will not kill them but it will have some sort of marring effect on them. It is forbidden for an Aedis to destroy the shadow of another person, and is generally frowned upon though not expressly forbidden to destroy the shadow of an object.
I'm going through now and trying to find what needs answers still, there were a lot of comments made in my absence.