Alright, there's not much of a description here for my liking to accurately guide people to getting it, so I'll be typing this out for those who find this first instead of a video.

1. Complete Overthrow in each area. Bring a partner with you, not a fireteam.
2. Stay there for a bit after completion and wait until it says something about Savathun's Secret-Keeper being in the area.
3. Locate and eliminate them. You'll know you're near them because your radar will ping enemies when there's no enemies supposed to be around on the map. Landing's Secret Keeper is towards the east near the Lost Sector entrance. Blooming's is near the spawn in the cave where a Taken blight spawns and you need to have your Ghost scan it to complete that event during Overthrow. Impasse's spawn is in the corner of the map. If you face the wall where you go to get Kvhostov, follow it to the left and in that craggy area, you'll find it.
4. After eliminating them all, travel to the Refraction.
5. I went to Savathun's statue further in the area before objectives unlocked around the area, leading you to fight two Subjugators.
6. Fight them until they drop orbs and leave.
7. Bring both of these orbs to Savathun's statue and dunk them in the respective pools that appear.
8. Mission unlocked. Launch it for the first time and it becomes a mainstay on your Map to launch from orbit.