How to get the Black Spindle exotic sniper rifle (an upgraded version of the Black Hammer it seems) was another one of the Taken Kings many secrets discovered by players accidently.While completing the daily heroic story mission "Lost to Light", the group discovered a secret a secret path that started a secret mission during the escape sequence of the original Lost to Light Mission.

Here are the step by step instructions for starting the secret mission and how to complete it. You can watch the original walkthrough of the mission here.

Note: The secret quest can only be run when the daily heroic story is Lost to Light. The regular version of the mission will not open the secret passage.

1. Assemble a fireteam. Seriously this mission gets pretty hairy and will take at least two players, but you should aim for 3.
2. Select the Lost to Light mission from the Daily Heroic Story selection on the navigation map. If it's not there you'll have to check back each day until it is the daily heroic again.
3. Run through the mission and when it gets to the part of the escape sequence unlock the two sealed relic doors but do not open the third with 3 locks. Instead head left after passing through the door with two locks and look for a passage entrance lit by a Hive crate. Follow this passageway until your ghost pops up with the new mission information. You may remember this area as thepath leading to Tanik's Ketch.
4. Follow the waypoints into the ketch but be ready. As soon as you arrive in the first room you will have exactly 10 minutes to clear the entire ketch of all enemies. This task will take a fireteam, so again make sure you bring some friends.
5. Be especially careful of the wizards which can rain down the pain in the tight quarters. Take them out as quick as you can. The final boss is also a tough hombre, so pack some extra heavy ammo synth to burn him down quickly.
6. The final boss fight is pretty straightforward. Have one guy focus the blights that will be continuously spawning in while the others focus on the boss. 
7. Collect your sniper rifle!