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 have access to Lost Prophecy Verses at the Weapon Forge in the Lighthouse. Some of them will require you to gather Fossilized Hermaion Blossom in order to fulfill the prophecies and forge the weapons.
In order to obtain these, you must complete Nightfalls, Trials of the Nine matches, or Heroic Strikes. The new resource has a chance to drop after completing them.
Fossilized Hermaion Blossom
As you can tell, these are some of the more difficult activities to undertake in Destiny 2. Fortunately, there aren’t very many of them that need to be done in order to get the amount required for whatever Lost Prophecy Verse you’re trying to fulfill. Once you have the required amount of Fossilized Hermaion Blossom, you can visit the Weapon Forge and forge your weapon with the fulfilled Lost Prophecy.
For more information on Lost Prophecies, their various Verses, and what weapons can be forged, be sure to check out our Destiny 2 Guide Hub.