Once you wrap up the Curse of Osiris story, complete the first three Adventures on Mercury and then repeat one of them in its Heroic variant, you will be given the Lost Prophecy, Verse 1 by Brother Vance. In order to complete this Lost Prophecy you will need a Concentrated Radiolarian Culture.
Brother Vance actually has this item, but will only give it to you if you can give him ten normal Radiolarian Cultures. In order to obtain these, you must complete Public Events. The new resource has a chance to drop after completing them, or from killing High Value Targets. They can also drop as rewards from opening planetary chests, a method of farming that can be much more efficient than completing public events. This isn’t mentioned by Brother Vance at all, it must have slipped his mind.
Concentrated Radiolarian Culture
To get your ten normal Radiolarian Cultures, you can do Public Events of kill HVTs on ANY planet. This is very important. Despite being attached to DLC content, the farming of the item is not restricted to Mercury. You can go to any planet to farm them. I strongly advise using whatever planet is hosting this week’s Flashpoint, due to the high likelihood of HVT spawns. It is also helpful to have a lot of people doing the events with you, to finish them up faster. In between events just clear off any region chests you’re still missing or farm planetary chests while you wait.
There does not appear to be an increased chance of them dropping from Heroic events, so if you want to just rapidly finish up the standard version, that is fine. For me, the resource dropped after roughly every second Public Event, with HVT kills dropping them at a more random interval. There also seems to be a small chance of getting a Concentrated Radiolarian Culture to drop, which is very nice.
Once you have your ten normal Radiolarian Cultures, return to Brother Vance on Mercury and trade them in for the Concentrated Radiolarian Culture.
Now that you have your Concentrated Radiolarian Culture you can complete the Lost Prophecy, Verse 1 and activate the Forge. One final note, these items only seem to drop if you have a Lost Prophecy from Brother Vance in your inventory, so it does not appear that you can farm them without one.
I hope you found this guide helpful, Guardian! If you need help with any other aspect of Destiny 2, including the new Curse of Osiris DLC, be sure to check out our Guide Hub.