How would you make an item that transports you into a separate zone? (Like entering a cave?)

[Evilkitty]

I'm new to modding and I haven't seen anyone else do this. If I could get a point in the right direction that would be helpful. I'm assuming the same mechanic as entering a cave could be implemented but I'm not sure what the limits to modding are.

On top of that, I'm planning on making this a custom character starting item. Could this item mod in the same mod folder as the character mod?