Currently, the most powerful items in Diablo 4 - the "Uber Unique" items - are extremely rare drops. They can drop from the world, or more commonly farmed from the Duriel, King of Maggots boss, at around a 2% drop rate. However, you cannot always get the piece that you want and may end up never getting it despite your best efforts as their drop rates outside of the boss are almost impossible to obtain.
Since Diablo 4 Version 1.3.2, players are now able to salvage these items to obtain the new Resplendent Sparks crafting item, and Transmute 5 of them at the Alchemist to create an Uber Unique item of their choice. Notably, drop rates for Uber Unique items outside of the boss has also been documented to be increased in this patch, with no exact numbers given. That is, until now!
Uber Unique Drop Rate
The datamine from the patch has revealed that the drop rates were increased by 1350 fold compared to previously. In summary, there is about a about 1 in 250 Unique items will be an Uber Unique. To find a specific Uber Unique item, it would be around 1 in 7500. So, it is still rare but now not as impossible.
Datamine Source
The datamine source, is from Lothrik, who created the Diablo 4 Build Calculator, one of the early tools that has been available since the game was in beta.
Rob2628, a well-known Diablo 4 content creator, covers this topic in more detail.
