I'm working on a zombie faction mod (see here: http://www.reddit.com/r/RimWorld/comments/25w2tu/my_new_zombie_faction_mod/)
And I would like to make backstories for them only. How would I go about doing that?
Take a look here : "RimWorld\Mods\Core\Defs\FactionDefs\BaseFactionTypes.xml".
If you're looking to make your own custom Backstories (i.e. not using the existing Backstory categories in the FactionDef), you may have to wait until Alpha 4.
Reading over the Changelog, I saw a bunch of comments a couple weeks ago talking about making Backstory generation more mod-able.
Creating a custom backstory is something on my to-do list for my K9 mod, but I just haven't had the chance to thoroughly investigate it yet. If I find a way to do it, I'll let you know.
Ah, I see, thanks for the help guys.