We now also got a look at one of the two mystery Tuning Stones, Evernight! The stones are used to upgrade your Seneschal companion, and you can find a list of all of them here.
Back in the Season 3 preview, there were two especially rare and powerful stones mentioned. However, we didn’t get any information on what they did. Now, in a PC Gamer interview, Blizzard shared a screenshot of one of those Tuning Stones, and it is indeed the equivalent of an Ueber Unique! 
Harlequin Crest, aka Shako, in fact!
Source: PC Gamer.
The other stone, Genesis, is still a mystery. However, Blizzard did confirm that it will also be of Uber Unique power level! We also got a look at a few more already revealed Stones in the actual game, with specific numbers, so head on over to the PC Gamer interview if you want to check those out.