Ok I don't know how to explain what happened but I'll try anyway, I was farming rubido on earth oro and I got a message pop up with a boss I've never seen before. He said "you will pay for your crimes agents golem" at this point the screen was flashing. I went up an elevator and there was a guy called 'the executioner'. I didn't even have enough time to printscreen and after he killed me he just sat down and disappeared.
If any one sees the executioner please send me a message on the warframe home page (drdoomv71), thanks.