[font="Arial;"]The interesting thing here is still Umbra, though. The Great Beings drained him off his inner shadow, and knowing the GBs, they wouldn't have wasted the opportunity.[/font]
But they had just created him, apparently, and endowed him with those things. You don't seem to be taking that into account.
Also, as I've mentioned before, I'm not sure we know that he ever had any inner shadow. We can't assume this is the case without confirmation but it would seem to be more efficient for them to just make him without any in the first place. So he demonstrates what it would be like if someone were to drain their shadow entirely, but that doesn't mean he ever had any to drain. I've so far been unable to find confirmation either way; only that the GBs made him and that he has only inner light.
Even if they did have to make him balanced and then drain shadow, since they can apparently make either at will, I don't see why they would be concerned about what to do with it.
And yes, I saw that you mentioned converting, but then you seemed to go on to speak as if it wasn't being converted to anything.
BTW, a simple analogy may be helpful here. When the batteries in a flashlight drain, where did that energy go? Simple -- it makes light, and light easily shines. So most likely any drained light just shines away. Most things in Bionicle with special effects produce light anyways (it's a common trait of such stories), so my guess is the Shadow Leech's "burning up" is what releases that light into the air.
Shadow energy might not move fast like light but can be released into the air in the same way (probably to be quickly cancelled out by any light that present).
Actually, just checked BS01's description, and to me it sounds more like a cancelling out thing:
Shadow Leeches fed on light, and could drain the inner light out of a being with a sufficient amount of light in them by physical contact, making the victim evil. After sucking out a being's light, a Shadow Leech would instantly digest and eliminate the extracted light. Once this was done, a Shadow Leech would typically disintegrate within ten minutes.
To eliminate light, they must have (under the wave theory) a lot of shadow energy to counter it. And probably only that. I'm unsure of the "fed on" wording, whether that means they needed it to survive, but too much of a good thing, etc. They may be modified in such a way that they can only keep the two separate if there's much more shadow. Ingesting enough light to drain someone completely could break that barrier and cause the two to cancel each other entirely out.
Then, the Leech would be just a dead body (invertabrate; a mutated Kraata slug). Beings need at least some of one or the other to survive (Umbra's page for example says that if his light was drained he would die since he has no shadow; apparently indicating that neutral "no wave" state). I can see how a mutated slug (perhaps drained of moisture as part of the mutation, so it's like a walking heap of dust held together by shadow energy??) could disintegrate quickly while dead. Jellyfish can fall apart in a similar way in strong sunshine and dry air.
So that may be what's going on, rather than the "shining" idea. Either would make sense, though.