The Spirit of Farradorn can be found by interacting with a glowing rock in Falador mines during the Invasion of Falador after following a trail started by finding the research notes.
Talking to the spirit unlocks a regular cosmetic version of the Banner of Farradorn, as well as a permanent bonus in the Invasion of Falador event. If you had assembled the 10 banner fragments first, which also unlocks the regular version, talking to the spirit blesses your banner.
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