Paint your wall or walls black. Get some 1/2-1/4 inch masking tape and mask off a random stone pattern. Depending on how big your room is and your budget get a 5 gallon bucket or more of drywall compund and a gallon of black or the darkest grey paint you can find, (the cheaper the better) mix this really good, don't be afraid of how dark it is it will dry lighter. Coat your wall with a good heavy coat of this, use your hands to apply it not a trowl or any tools. While it is still wet, (work in small sections as you go so it doesn't get too dry) peel off the tape to reveal the black lines. let it dry really good, it will crack, but thats good makes it look even more real. Then dry brush it with lighter shades of grey and black to make it look lke real stone, (don't over do it...less is more) This stuff is called Monster Mud and we use it for everything in haunts. You can even use some flouresent paint to highlight some of the stones around the edges, (don't paint over the black lines completly, add a black light or two and admire your work, (use the blacklight while you work to help you with your pattern. You could draw in a fake window and have a castle scence or anything else you like.
You can also make stone walls using sheets of builders foam, and it looks great, but thats time consuming and a lot more costly.
Try this for some pattern ideas:
http://www.unclebinleysroom.com/dung...Walls_Gallery/