Uroboros

Uroboros is a Boom wad with non-linear progression between levels. Each map has both normal and secret exits, leading to different next map. But there's more! It may also be absolutely random which exit you will get (however you might discover secret deterministic exits), so there are many different ways to progress through the wad. Don't be surprised that you got into map 8 after map 1. It is completely normal :)