The Pursuer

The Pursuer is a mob that you will encounter as soon as you enter the world.

When it appears, a message will inform you in chat.

It has two forms: One where he attacks you in melee and another where he flies and attacks you from a distance.

You can actually flee from the Pursuer while it's changing forms. Fighting back is useless since it's basically immortal.

Your best option is to hide in caves or in other dimensions.