Ender Danger

The ender danger is a new miniboss in the end dimension. It looks like a phantom, but it has more health, it deals unforgiving damage and it's fast. 
It lives in the void, so you'll have to pick an elytra fight with it.

The ender danger swoops down at you like a phantom.
It also has two special attacks:
1. The stealth attack: it goes invisible for a few seconds, and it shows up again when it's close to you.
2. The scream attack: it screams "danger danger!!" in an encrypted Minecraft language, creating a sound wave that damages the player (and mobs).

Spawn location
It spawns alone in a special structure, a set of few floating glass islands. 
The glass islands have loot, but you can't immediately land to pick it up because the ender danger would knock you into the void.
So, you'll have to kill it before taking the loot.

Reward
Killing the ender danger spawns one teleportation block on the glass islands. You can mine it to get it in your inventory.

When you click on it, it takes you to a GUI where you can choose to teleport to any other teleportation block in the same dimension. However, every teleport will cost you an eye of ender.