Reposting what I saw on Reddit:
https://www.reddit.com/r/elderscrollsonline/comments/6so3y0/the_emperor_title_is_one_of_the_rarest_titles_inHow it works: Whenever you're crowned as Emperor in a campaign, you're given the Emperor/Empress title. Whenever you lose emperorship, you lose the Emperor title and are given the Former Emperor title. However, if you're the active Emperor of a campaign whenever it's removed, you can keep the Emperor title forever. This works because, technically, the campaign you're Emperor of still exists, but since it's inaccessible nobody can get in there to dethrone you. This is commonly referred to as "permanent emperorship" or "perma emp" in the community.
Since campaigns aren't closed often, and not every campaign has an active Emperor whenever it's closed, this makes the Emperor title extremely rare, with less than 15 people on PC NA with the title. It's so coveted that on PC NA we've actually had people offer to pay hundreds of real-world dollars for it in the past.
How to get it: The campaigns Xarxes and Kastav were both introduced to handle population overflow from the other campaigns during the Midyear Mayhem event, and will be closed when the servers are taken down for maintenance for the Horns of the Reach DLC this early Monday morning (~4 A.M. EST) or Wednesday if you're on console.
What you need to know:
(These times are all for PC NA, and may be different for different megaservers)
Xarxes' leaderboards reset at 3:37 P.M EST Thursday (that's tomorrow)
Kastav's leaderboards reset at 12 P.M. EST Friday
You'll want to start playing as soon as you can after the leaderboards reset so that you've got the best chance of being #1 on your faction's leaderboard.
Remember, to get the Emperor title permanently, you need to be the active Emperor of that campaign whenever the servers go down for maintenance (usually around 4 A.M. EST Monday morning.)
Full disclosure: I had initially planned on going for perma emp myself this weekend, but unfortunately I wasn't able to get off work. I don't want to see this opportunity go to waste, though (remember, campaigns don't close often!) so I figured I'd post here.
Q&A:
Q. Is this exploiting?
A. No, it's not. The Emperor title has worked this way since the game was first released on PC back in 2014, and ZOS has been aware of perma emp ever since. Since it's a purely cosmetic bug--and one that encourages PVP--they don't have any interest in fixing it.
Q. If I get perma emp, will I get to keep the emperor buffs forever?
A. No, you only have the emperor buffs in the campaign where you're currently emperor. You do, however, get to wear the Emperor title in every campaign, and outside of Cyrodiil as well.