This Triumph requires you to complete Master/Grandmaster Nightfalls, dungeons, raids and/or win rounds of Trials of Osiris while wearing a complete set of fully upgraded (Glowing) Solstice armor.
I have learned that completing the Duality dungeon DOES NOT give progress or Silver Leaves.
Most likely because of a bug (#PulledABungie) & I do not know if this issue applies to the other dungeons.
I can confirm however that Raids and Trials wins DO give progress and Silver Leaves.
More specifically, a single raid completion on normal difficulty start to finish or winning 3 rounds of Trials will complete the triumph. As for Master/Grandmaster Nightfalls, I do not know.
That's all.
Good luck gilding your Flamekeeper titles Guardians! :)
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Done with getting my seal.