Hi The_Cannibal. The Mummy of the Ring is not currently implemented in the game for some reason. Word around the campfire is that he will be released as add on content at a later date. It is possible to spawn him in using the admin panel, he feels very weak and unfinished. But I think that you were already well aware of this, and are hoping for some actual news on his release right?
I have always viewed Journey Steps as being the closest thing we have to a main questline in Conan Exiles, and as such would really like to see them all functional, and up and running. And as far as I am aware, this is the only one which still cannot currently be completed (well without Admin anyway).
Ok there is no official word yet. However a few recent scraps of information have come to light which have given me hope that he is being worked on. This is pure unadulterated speculation, so take it with a pinch of salt, strap on your tinfoil hat and follow me.
Here is a recent comment made by a CM when reporting his absence in a Thread. Here is what was said:
Hello @Scumdog1980, thank you for reaching out!
Regarding this matter, our team is aware of it and considering options to finish the journey steps
So it does sound like something is being done, or at least considered to get this Step sorted.
Here is the second thing I noticed in the official Gamepedia’s upcoming features section Here. It basically says:
Update 40 or higher (ETA: H1 2020[12])
*Possibly includes a continuation of the story, as was previously mentioned to come in 2020
Yep, really clutching at straws I know. But retrieving the 7 Artifacts and finding a way to escape from the Exiled Lands pretty much is the main story. So fingers crossed hey.
But one thing you can rest assured of is that I am really going to keep on Funcoms case about The Mummy of the Ring this year (in the nicest and most polite way possible of course 
). It has now been 2 years since the game launched. I would really like to see the games main questline fully implemented and completable by all players.