I am a DM with but a few games under my belt so I ask the good people of these message boards if I have a good idea or a horrible one. Ravenloft and Darksun. Two very different worlds but with one thing in common. The fearful thrill that runs down every players spine when they hear their character is going there. What I propose is a new world for my players that is a meshing of the two. The limited races but occational wierd outsider showing up of Ravenloft with the slave owning savage and often evil denizens of darksun. an intense fear and hatred of arcane magic. The defiler rules. Outsider ratings, dark powers, limited living lands, and a dying sun that is truly dark giving only low light in mid day. Comments are appreciated. Thank you.
Ravenloft is Hammer Horror world and Darksun is 80's direct-to-video fantasy/post-apocalypse, two great tastes that have nothing in common apart from the occasional undead wizard. So I'm not sure what you're looking for here.
Are you trying to bring Madness saves to Darksun? Have one of the Sorcerer-Kings turn out to be Lord Soth?
Dark Sun and Ravenloft have something in common: Kalinday, a city-state from a Tyr region that became a Domain Of Dread. It was described in one of 2nd edition books. It also contained confusion over the gender of it's (dead or comatose) Sorcerer-King (Raveloft book suggested one gender, Dark Sun books suggested the opposite).
It should be possible although you would need to do significant changes. After all, horror can be set anywhere, and there is already some overlap with Kalidnay (as Drejk already pointed out). Similar themes: a downtrodden common populace ruled by very powerful individuals, fear of the arcane and maybe the supernatural in general. A hard life constantly under threat, both from danger within and without, xenophobia and superstition. Lack of control is a common theme, too.
I would probably start by merging the Amber Wastes with Dark Sun domains. Sorcerer-Kings should often be Dark Lords. The general horror theme would be "terrible secrets buried in ages past, now unearthed"; think of the classic horror mummy. You can mix things up with stuff like civilisation-threatening curses, for example swarms of locusts or even worse creatures that will destroy the last sources of food/water, crazed serial killers in the big city states and other staples of urban horror.
I guess that it would be a rather limited setting, to be honest, as the cross between Dark Sun and Ravenloft makes for a very specific theme. Calling it Raven Sun is better than Dark Loft, of course ... ;-)
Thank you all for the information. As for what I'm trying to accomplish: to fix my lack of knowledge. When I was discussing my new game idea and mentioned, what I thought was an original idea of having intelligent insectoids, my three veteran players highfived and shouted "Thri Kreen!" sheepishly I had to ask what that was. This led to a long conversation. Then I started talking about the feel of the game and how races interact with eachother mentioning again I thought an original idea of having different races being xenophobic and having them very superstisious of magic. To which I got the question "Wait are we in darksun or Ravenloft?" I've been researching since, but I know those three know far more then I do because they have played since 2nd edition. I like both settings but would be horrible with fear checks. My players are far too decensitized for me to scare them. The struggle of both worlds are appealing, one to stay alive one to stay sane, I'm unsure where to go from here.
If your players know about the settings and you do not (as much), it would probably be better to build a setting from scratch and just take inspiration from Ravenloft and Dark Sun. Otherwise they might be put off by stuff they recognize and remember from another background. It might even be better to change the general feel of the setting somewhat, in order to keep it separate. For example, choose neither Dark Sun's deserts nor Ravenloft's often gothic horror eastern european (yeah, I know, Raveloft has a hodgepodge of domains, but this one is the most evocative) as the underlying theme.
You can find rules for sanity here. They are from Unearthed Arcana, which is D&D3.5, but they can at least provide some ideas. While your players might be jaded, they should in time become afraid to lose their characters to insanity, which can be as bad as dying - or even worse.
Thank you all for your help. After careful study of each system I find I'm more suited for Darksun. And like the rules found in athas.org except one. The defiling magic. I'm not sure if I should add to the dc like suggested. Go back to 2nd eddition and have it memorizing spells or have it be when they cast spells it does different things and potentialy hurts other players. What are your thoughts and any fun ideas for what defling could do to just make the party hate the wizard