Well, have you?
If you have, then this post on the Abaddon Books website will be of interest to you.
The Dark Overlords of the Pit (a.k.a Abaddon editorial) are looking for novella submissions set within one of their existing shared-world properties, or even ones set within a brand-new world of your own creation.
To find out more about this amazing opportunity, follow this link.
However, it is worth noting that Abaddon Books operate on a Work For Hire basis. Abaddon retains all the rights to those worlds and characters that appear in the stories they publish. If this doesn't work for you, then try elsewhere. But if you're happy with that arrangement, then this could be the launch pad to who knows what...