Here is the next video in my Super Meat Boy playthrough. In it, I get an A+ on all the levels in the dark world of The Forest chapter.
The dark world contains more difficult versions of the levels in the light world. You unlock each level by getting an A+ in the light world version of the level.
The achievement for completing all of these levels without dying is the Squirrel Boy achievement. I was able to get the Wood Boy achievement in the light world but still don’t feel confident enough to try for the Squirrel Boy one yet. You will see in this video that I need to practice before I can get it.