I discovered this magical secret by accident then tested several times to find the exact pattern. Now I present how to reach the 'old' Forest Haven campaign!

Obviously the key is in the Shadow Caves (I always knew that place was full of secrets). In the fifth cave (it doesnt matter how you get there) you must clear the whole cave and my dear friend mr Vyxnaar must be there. Save him as the last kill, get him down to almost dead, and the final blow must be made by a mega combo (any of the three works). BUT heres the catch!-everyone in the party must be wearing a vyxnaar helm when he dies (again any of these three will work). Then with a flash of smoke, light, and mirrors(not really) a magical portal appears entitled the 'Anti-Shadow Portal'. Stepping through it, you will find yourself in the ancient FH1 map with the infinitely spawning zombies. Totally worth the difficulty IMO. Enjoy!