User blog comment:BobTheDoctor27/April 2013 Update/@comment-5918010-20130517002454/@comment-958607-20130517230929

OK, I haven't been into much detail in regards to the surface of Bara Magna in the Fractures Universe, but there are relatively few Toa there. There certainly wouldn't be any that far north as the Brotherhood hasn't ventured past Iconox. (See most recent chapter of Frozen Calling)

I think it would have to be the other way around. A Matoran of Shadows cannot become a Toa. Only worthy Matoran can, so he would've had to have been corrupted. Additionally, this seems far too complicated and, really, doesn't have to fit the Fractures Universe. The only possible reliance he could have on it would be getting bitten by a Shadow Leech, which could happen in any Universe.

I would encourage you to create your own alternate universe, which will give you far more freedom to do what you want, and to sculpt the story to your own desire. :P