Alright after some time of playing this wonderful game i am interested in playing in some more varied and crazy worlds. My question is how the heck you would go about making for example a half frozen world dotted with volcanoes the other half being maybe an evil forest with killer unicorns and other weird wild beasts? I guess maybe the better question i should ask is if there is a guide somewhere on how to make custom worlds.
That's certainly possible. If you have a specific set of requirements, feel free to post them in
this thread, and myself and others will do their best to provide an ideal world & embark.
However, to answer your question specifically, you'd reduce the temperature range to -75/75, increase volcanism to handle however many volcanoes you want, and increase good and evil values for the region sizes that are typical in your world. Region sizes vary based on elevation, temperature, and biome features, and can be customized based on Variance range. Put simply, the higher the Variance, the smaller the regions, if the combinations are sufficiently aligned.
Currently, to the best of my knowledge, there is no utility or tool that previews Evil biome effects (such as blood rain, doom clouds, re-animation, husking, zombie-fication, spawned Corpses, etc) prior to embark. As such, if you truly want specific types of undead, a bit of trial and error is required at the moment.
This world, though, has much of what you're describing, so you can examine it to see what parameters were changed.
As far as a guide goes, the wiki has
some details.
Finally, keep in mind any world that generates with rejections will NOT reproduce, cross-platform. This only matters if you intend to share your worldgens, or are using other peoples worldgens. If there is even one rejection on a world generated in Windows, it will
not be the exact same world that appears in Linux or OS X, for example.