Diablo 4 players found Annihilus in Season 15, but nobody has solved the trigger yet
The patch notes never mentioned it. Players found it within hours anyway.
One of Diablo 2's most famous items has officially made its way into Diablo 4! Annihilus is dropping in Season 15, Uber Diablo is walking Sanctuary again, and Blizzard apparently decided not to tell us about any of it…
Players started finding the Unique Horadric Seal not long after Patch 3.2.2 was released on September 30. The funny part is that neither Annihilus nor Uber Diablo appears anywhere in the official patch notes, and the community had to find them the old-fashioned way instead.
Diablo Really Does Walk the Earth Again
The first confirmed 
Annihilus drops came from a secret Echo of Uber Diablo fight appearing after World Boss kills. Once the World Boss is dead, there is a chance for a very familiar message to pop up:
“Diablo Walks the Earth.”
Uber Diablo then appears, and killing him can reward Annihilus.
It is about as Diablo 2 as this could possibly get.
Multiple players have now reported the same sequence, so the World Boss connection looks confirmed. What is not clear is what makes Uber Diablo appear in the first place though, because he does not show up after every World Boss. Various players have killed multiple without seeing him, then suddenly getting the encounter on a later attempt.
And that leaves us with one rather big mystery.
Stone of Jordan Is Looking Very Suspicious
If you played Diablo 2, you probably already know what we are about to explain.
Diablo Clone was originally summoned after enough Stones of Jordan were sold to merchants. Once the required number had been reached, players received that famous “Diablo Walks the Earth” message and could hunt him for Annihilus.
The 
Stone of Jordan also returned in Diablo 4 Season 15. So naturally, people started experimenting.
Datamining discussed by the community indicates that Diablo 4 keeps track of salvaged Stones of Jordan, which would be an oddly specific thing to count for no reason. However, players had already been investigating that connection before the first Annihilus dropped.
But there is an important distinction here, because we still do not know if salvaging Stone of Jordan is what triggers Uber Diablo.
At this point, all we really know is that Uber Diablo has appeared following World Boss encounters. The hidden condition deciding when that happens is still being investigated.
So if you were thinking about salvaging every SoJ you have, perhaps hold off on sacrificing every ring in your collection just yet.
Annihilus Can Give You +10 to Every Skill
The reward itself certainly lives up to its Diablo 2 name.
Annihilus is a Unique Horadric Seal with six Charm slots and can roll:
+10% to 20% All Stats
+10% to 20% Resistance to All Elements
+5 to 10 to All Skills
+5% to 10% Bonus Experience
Yes, the Seal really can give you +10 to All Skills.
Annihilus can also roll two Seal Powers, so finding one is not necessarily the end of the hunt, as you still want a version that works with your Talisman setup.
And even though those numbers look quite ridiculous, Annihilus is not automatically better than every Mythic Seal. Something like 
Seal of the Diamond Mind can enable Talisman combinations that Annihilus cannot, so the best choice will depend on your build.
Regardless, +10 to every skill is definitely something to get excited about.
One Last Thing: Check Your Loot Filter
Before you head off to camp farm World Bosses, there is one more thing you should know…
Annihilus is a Unique, not a Mythic.
It would be very easy to overlook the drop because it looks like another Unique on the ground. Depending on how strict your endgame loot filter is, you might even have Uniques hidden entirely, so make sure to check that beforehand.
If Uber Diablo does decide to visit your World Boss instance, it might be worth disabling the filter before killing him.
In any case, the hunt continues. We know Uber Diablo is real; we know he can appear after World Bosses, and we know he drops Annihilus.
What we still do not know is what makes Diablo decide to walk the earth in the first place.