I also thought this looked the Horadric Cube, that's why I chose it as an avatar.
But the name it's so abstract for a reason.
A person that I had met not a lot of time ago, who was friend of one of my best friends and players, called me a "Diablo unique item" as a way to say that I am a special person in a positive way, after having GMed a game for a group I was in.
This guy is one of the best GMs I had, even though I could only play with him once, and also a wonderful person. Receiving a compliment like that from someone who barely knew me really touched me.
So a few years later I still use "Diablo unique item" as a joke title.