I have been playing Terraria for a couple of years now, and every single time I think I've finally experienced everything it has to offer something like this happens! While the previous cases of 'something' were usually official updates, this is going to be the very first time a mod has intrigued me so much that I'd be willing to jump back into Terraria for it.
The mod in question is Super Terraria World - a standalone RPG mod that adds new skills, quests, NPCs, items, gameplay mechanics, as well as a whole bunch of new and expansive worlds to explore. Since all of this sounds too good to be true, allow me to show you the recently posted trailer as it is pretty damn impressive. Have a look:
Now before you get too excited, its important to keep in mind that Super Terraria World is still currently in alpha, and as such some features and gameplay elements are missing. On the positive side, however, most of the singleplayer elements, environments, and things you need in order to actually have fun with it are fully present and accounted for!
You can download Super Terraria World by heading over here, and don't worry about the installation process - it only took me a couple of minutes to get the whole thing running. If you do end up having some trouble, worry not as the developers have also released a brief tutorial to guide you through the whole thing. You can find it here, but before you head out, do make sure to back up your current save files. Just in case!
And finally, if you're interested in learning more about Super Terraria World and its upcoming features, I would suggest you head over to the official Terraria forums. Have fun!