You have to go flawless in trials of osiris on the VoG map and then you get access to the deathsinger boss. You then have to face her in a 1v1 trials match while high on cannabis. She will flee and then you have to do the crotas end raid on the secret difficulty and kill her for a chance to get the vex core. You then bring the vex core and a strangers rifle to the gunsmith who will then tell you get 420 postmortem kills in the tower. You then go back to the gunsmith and kill him to obtain your new brand spanking vex mythoclast.