This is Wynncraft, a gorgeous fantasy MMO that you can play on a Minecraft server.
It's a full and proper MMO: Wynncraft has everything from quests to levelling to guilds to bosses, and takes place on an enormous map that's around 4000 x 5500 blocks in size (you can view a zoomable map of the whole thing here).
Wynncraft's team has been working on it full-time for four years now, and the results speak for themselves. This map would be amazing just to walk around and explore, let alone having it be the foundation of an online RPG.
Wynncraft is free to play so long as you've got a copy of Minecraft, with the team making enough money to pay for the servers (and then some) by selling items for the game.
If you want to try it out, there are server details here. And below is a trailer for the latest update, so you can see what it looks like in action: