Who is this Nemisis monster? [duplicate]

While playing Diablo 3 today, I encountered an Elite monster named Nemisis several times. He popped up very randomly. I noticed he had a rune-like symbol under his name as well.He popped up twice in Act 1(I don't exactly remember what I was doing those times) and once in Act 2 (right before I went into the Witch's Den to fight her). He showed up probably a few hours in between his spawns. After being pulverized by him the first two times, the next time he showed up I was able to kill him. But an even stranger thing happened when I was about to kill him, as he had several shadows of myself attacking as well. He ended dropping a good bit of loot including a legendary item. What I want to know is his characteristics, like those rune-like symbols were, what triggers him to pop up, and anything else that would be helpful to know.