Era

Warning: here there be spoilers!
_{To avoid spoilers, only highlight the text under headings for chapters you've read. Of course, if you don't care about spoilers...}

For all the profound (negative) influence Era has had on the Digital World, very little is known about him to the masses-- that is to say, nothing. Only his highest sergeants are allowed to see him, and even then they are expressly forbidden from divulging anything about him. He appeared 100 digital years ago (10 years ago in the real world), and instantly asserted his power, taking over much of the digital world.

As of Chapter 8:
Era is known to be a refugee from an undefined alternate universe; Wisemon's theory is that he was forcibly ejected from said world to prevent him from causing more damage to it. His power is unlike any digimon or human seen thus far.
According to Wisemon, he seeks to overthrow the Sovereigns and access Yggdrasil in order to use the latter's power to cross over into the Real World, using the digimon he controls as brute force to take the Real World over.

As of Chapter 22:
Era is human-- at least superficially -- and has appeared to the group. Human or not, he possesses the ability to stop six Perfect-level digimon with the wave of a hand and effortlessly strips Forbidramon of his power.

As of Chapter 41:
See here-- but be wary of massive spoilers!