Haha, the process involves Atheon
Destinyplayers recently found a secret in the Not Forged in Light Daily quest, but nothing could be done about it — until today that is, when the daily has arrived yet again. If you finished it before and picked up the extra Ghosts, you’ll get a quest, which will allow you to re-enter and trigger the questline for No Time to Explain, an Exotic 290 pulse rifle.
Uncovered by reddit, all you have to do is go back in and grab all three hidden Ghosts in one run without wiping. To get the third Ghost, after you kill the Templar boss from the mission, do not finish the last bit in the Gorgon treasure room — instead, head back down to where you would fight Atheon, by way of the jumping puzzle. Head left into the cave, then drop down to a ledge on the very bottom, to the right. That Ghost will trigger a new jumping puzzle to a portal on the left. Beating the miniboss inside will trigger the rest of the quest.
In essence, you’re going to have to kill Atheon again (grab a checkpoint or just do it on normal), then do two quests. After that, voila, the weapon is yours. It’s pretty easy all things considered, and it only took about 30 minutes for me to do earlier today. If you don’t have the quest yet, now is a good time to get it started, as you’ll have to wait until the next time the daily pops to get the weapon.