The Symbol of the Guardian Brotherhood is a legendary crafting material used by Blacksmith Haedrig Eamon to craft the Guardian’s Jeopardy set. The Symbol of the Guardian Brotherhood drops from Dragus (demon trooper) in Act III: Stonefort.
The Symbol of the Guardian Brotherhood also drops from Busaw in Act III: Rakkis Crossing during the Crazy Climber event (Bring the spyglass to Lieutenant Gryffith) — which starts with Private James.