Shade

Shade is the "final" boss found after defeating every other boss in the game. The Wither is, somehow, not required to find Shade.

Shade is split into two halves, the slightly angelic "Light" half, the one that helps you fight the slightly demonic "Dark" half. The "Dark" half is the real boss.

To locate Shade, you'll need to create a large portal filled with water and lava with one block from every dimension in no specific place. For example: Dirt - Overworld, Netherrack - Nether, End Stone - End, etc.

In order to make entering Shade's dimension easier, all you have to do is make a line out of them, instead of making a proper portal. Once you have all the materials, it will immediately turn into a giant chest. It will jump VERY far up, about 192 blocks, and check if there are any blocks blocking it. All players playing will get a menu where they can choose to help fight Shade or not to.

The half-heart gem does not affect Shade fully, but instead makes all of his forms have 80% of their normal health, but his attacks will be 5% stronger.

Tactics
After jumping into the giant chest, it will close behind you. A wolf will appear in front of you, and attempting to hit it or tame it will have a "No." approximately half a second after the action.

(Shade) Well, I'm the bane of your existence! (Player) Uh- (Light Shade) He's the "bane of your existence". Not me.
 * Shade splits into two*