Diablo 4 originally launched with a set of powerful 'uber' unique items that were so rare Blizzard had to come out and confirm that they were real. Only a handful of people in the world ever even saw one, which meant they effectively didn't exist for most players.
Even though Blizzard has been steadily making 'uber' uniques, now known in-game as mythic uniques, more attainable over time, they're still a chore to find. To raise the odds, you have to gather up materials to summon and defeat...