Xylar. You have to tea bag him and send a trash message within ten seconds of the kill, then you have to dismantle 56 year 3 primaries. Return to the tower and the cryptarch gives you 7 rare engrams if you open them in the correct order it opens a secret room beneath the vanguard upon entering there is an alter, you must sacrifice 10 kinder guardians and buy a £250 dlc which has a 1% chance of being vex Mythoclast. Only cost me £2500. Bargain.