A lot of people are speculating around the identity of the jailor. I will add my own over here for the sake of speculating and fun… Feel free to share your thoughts.
Looking at wow’s cosmic powers map. At the elements level you can see 6 elements (water, air, fire, earth, spirit, and decay). So far we have see 4 out of 6. Please bear in mind that the elements existed before the titans (as they are the building blocks of all physical matters). The only elemental lords that we didn’t see yet are Spirit and Decay.
The term jailor was only used in Quest logs were only with the Lich kings Quests, the one where Bolvar became the Lich king and the second is with Bwonsamdi questioning the Lich king about the identity of the whisperer.
Having said these facts about the lore, my theory goes as follows. Let’s look at what is common between the Lich King(s) so far in the story, the only constant so far is Ner’zhul. Ner’zhul is a shaman… A very strong shaman whom with time was able to communicate with the “decay” elementals, after having to the Lich King’s powers from the burning legions pact.
Ner’zhul has become or merged in some way with the decay elements and become like a decay elemental lord if I may say. This would explain
(1) the similarity between the jailor’s crown and the Lich king’s.
(2) What was communicating with Sylvanas when she jumped from ICC as she is greatly connected with death/decay. As she had a strong connection with death.
(3) The Jailor being older than the titans (a decay elemental lord?) As mentioned at the Blizzcon What’s next panel.
Having explained the decay element in my theory, the other element which needs to be explained is the the spirit element, hence comes Elune. There not much facts around Elune other than it communicates heavily with Night Elves priests (priests are heavily dependent on spirit) in particular. Having this strong connection with the spirit element gave the Night Elves priest like powers (like Tyrande being a high priestess).
I believe in this expansion we will get to see Elune and how she is connected with the spirit world (Shadowland). She will be our key/bridge to go there without dying.
Please tell me if you agree or disagree with me and why I know it’s all fantasy, but please be logical in your taking of your theory.