Much like the Sleeper Simulant before it, a new exotic weapon called No Time to Explain has appeared in Destiny today for followers of an esoteric, hidden questline. Players have been slowly uncovering the pulse rifle since The Taken King's launch by finding hidden Ghost Shells throughout the Daily Heroic version of "Paradox," a mission set in the Vault of Glass. If you managed to find those Ghosts and defeat the optional boss they unlock, you'll have access to the next step in the questline today.
That questline involves completing a few odd jobs for the Future War Cult, one of Destiny's in-game factions, including hunting Taken Minotaurs and fighting Atheon, the original boss of the Vault of Glass. After completing the quest though, you're rewarded with a 290 attack version of No Time to Explain, which can be fired in full auto mode and automatically adds precision hits directly back into the magazine.
The weapon sounds like a real boss-slayer — but if you haven't picked up those Ghosts already, you won't be able to complete the rest of the quest today. Check out this Reddit thread for more info on how to get your hands on this new Exotic.