In a similar fashion to the Secret Cow Level, put them in the Horadric Cube and then transmute them to create a Portal teleporting you to a place where you would have to fight the agents holding the Major Keys. Now with the Minor Keys set complete, you must now go to Act V, Harrogath. They drop it quite rarely, so this can take a bit, but keep going and you should get them eventually.

You have to defeat them playing in Hell difficulty, with a small chance to drop the Keys of Hate, Terror and Destruction respectively. The first step involves defeating the agents in possession of The Minor Keys, which are required to open the portals to the next set of keys. To stop this evil scheme you need to collect several keys that can help you enter Uber Tristram, where the Uber Prime Evils wait. Once Diablo regains his full strength and comes back to life, they decide to revive the Prime Evils, as well as Izual and Duriel. Lilith, the Mother of all Demons, shows up after the death of Andariel, her daughter, and the victory of humans defending the Worldstone, and revives Diablo by performing a Dark Summoning.

+ 2 Diablo 2 Resurrected Pandemonium Event - Overview & Details

Uber keys diablo 2