I'm considering making a modpack. One idea I'm mulling over for it is starting the modpack in media res with a boss fight. Now of course some considerations will need to be made, such as considering game loading lag and lack of weapons, armor, or resources to make weapons or armor. But right now, are there any mods out there that allow for custom bosses? Another alternative would be to maybe use Custom NPCs, but then I believe a preset world will need to be added. Is there a mod or technique for that? Or better yet, is there a mod which will allow for preset dimension at least? That way, instead of making the entire world preset, I can put the NPC in a custom dimension maybe using RFTools, Mystcraft (if I can every get it to work), or some other mod, and then spawn the player in that dimension. Thanks!
After many google searches, I managed to find out about a mod called Lockdown. Lockdown allows you to make a default world, lock the gamemode, disable the more world options button, and remove the multiplayer button. It is the answer to the latter part of my question, and hopefully this thread will be easily found by the next person looking for a mod like this.