I would like to get a copyrighted song, convert into a 8 bit song and use it in my game. Is that legal?
I am not a lawyer.
In most cases: nope.
But to be more clear, we need to know what is the license that the song was released at. When the publication has only
copyright (c) bla bla 201X, usually means that only the publisher has rights to distribute and/or make derivatives.
Sometimes someone release their artwork under the terms of one of the Creative Commons license (https://creativecommons.org/licenses). There are multiples of them. Some allow derivative work. Some allow free distribution. Some allow commercial usage. Some doesn't allow commercial usage. That is, some are more permissive, others are less.
Then there are the cases of sampling. Which is a gray area, but still risky. But, you said convert, which means maintaining the melody. That's much more risky than sampling. You still see a lot of people doing it, but you should be aware that people doing it are doing at their own risk.