The Emperor skill-line is part of the "Alliance War" category in the Elder Scrolls Online. The Emperor skill-line only has passives, very powerful passives that increase the offense and defense capabilities of your character by a huge amount. After all, you are the Emperor and you should be the most powerful player in the campaign!

Each Campaign in Cyrodiil in the Elder Scrolls Online can have one Emperor. That means there can be multiple current Emperors currently but they are all in different campaigns. 

If you are becoming an Emperor the first time, you will also gain an unique costume called "Emperor Regalia" and the title "Emperor". When you don't hold the Emperor status anymore, the costume will still be accessible to you, but the "Emperor" title will turn into "Former Emperor" title.

To unlock the Emperor skill-line your alliance has to capture all six keeps around the Imperial City in Cyrodiil and you need to be the highest ranked player in the current campaign. That means you need to have the most Alliance Points out of all players in your faction to be crowned Emperor.