A suggestion popped into my mind as i read the Seeker's mechanics. Give them a sanity meter and have it go down (maybe % based) whenever they chance upon forbidden knowledge. If they got down to 0, either auto recruit them to the player, have them become madmen who try to kill anything and everything, have them die or have them turn into something like a Cultist who tries to actively help you, though you shouldnt be able to enthrall them because they aren't enshadowed and blind to the darkness but rather acknowledge it and try to help it spread, so they still keep their mind, they just want to help the other side.
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The seeker is already a player-controlled agent, but the idea of having hostile agents turn insane due to what they learn is a good one. Maybe the seeker could choose to leave some of the clues they find behind, so any investigator who investigates the clue slowly goes insane. The agents already have a sanity value, because they have a full person, but currently that sanity value does nothing. With just a bit of changes (and the UI has space for it), this could be a different way to defeat your enemies, or sow confusion inside their ranks.
I'll see if it can fit into the next version.