You have just met the Stalker, the specific details of this character are unknown to me, there have been a few threads about it in the forums. Just search for "stalker" and you will see.
The stalker is believed to be a rouge Tenno that will attempt to assasinate other Tenno for taking down bosses. As far as I know it is a random thing, like a roll of the dice. You can defeat him if you catch him fast enough. He uses a combination of powers that resemble a mix of Excalibur and Ash's abilities. I myself got taken out by him once by a slash and dash strike, he pretty much one shot m