Everyone in Diablo 4 season 3 will get their very own robot companion that you can equip with all sorts of powerful skills and then modify with "tuning stones". You can read through a list of these new seasonal items in Blizzard's blog post, but two of them are left blank and said to be talked about "in hushed whispers" by Sanctuary's scholars.
In an interview with PC Gamer last week, Blizzard said these two companion tuning stones will mirror the power and rarity of Diablo 4's Uber Uniques and will give you "bragging rights" if you find them.
In the current season, Uber Uniques are the primary target for anyone powerful enough to kill the hardest endgame boss who drops them. They are so sought after that an entire economy for buying and selling the in-game materials to farm them cropped up on Discord servers and third-party websites for both real money and gold. And players regularly organize groups to share boss attempts, which is why you'll often see "LFM Duriel rota[tion]" spammed in chat [[link]] while you're playing.
Systems game designer Sean White wouldn't tell me how these Unique tuning stones work during the interview (Blizzard provided a screenshot of Evernight), but emphasized their power compared to the other 25 stones: "Let's just say this, if you were to find a unique tuning stone that is, in and of itself, bragging rights … because it is going to increase either your own power or the Seneschal's like—let's just say a lot. A lot, very many a lots."
Both of these Unique tuning stones, Evernight and Genesis, work like the others and change how your little spider robot's skills work. "Governing stones" grant your Seneschal basic skills, like the ability to fire lightning bolts, and tuning stones can alter them to shoot more projectiles or trigger effects you can build around, like stuns and poison damage. When White says they're equal to an Uber Unique, it means they'll give you something distinct from regular old Legendary gear, like a random Shrine buff for killing powerful enemies. Everyone will be chasing after Evernight and whatever Genesis turns out to be.
Uber Uniques used to be way more rare when Diablo 4 launched—like "only a few people in the world have them" kind of rare. Season 2 made them farmable with an estimated 2% drop rate from Uber Duriel. I managed to collect most of them after hundreds of attempts and can attest to their power. I was crushing demons as if I was playing on easy and it gave me a good point to start winding down my time playing the season. Once you've reached the pinnacle of power, there's not a lot left to do.
But later in season 3, Blizzard plans to add leaderboards for racing through a fixed dungeon that changes every week. Anyone who wants to seriously compete for the top ranks will surely want to hunt these new Ubers down. Blizzard hasn't confirmed where you'll be able to find them, but my guess is you'll have to grind the new seasonal boss, Malphas.
Diablo 4 season 3, the Season of the Construct, launches on January 23 at 10 am PT.