Padan Fain’s power has morphed through time, going one way and then another (Shadar Logoth to the Dark One and back), which is why Moiraine was able to sense him at the end of The Eye of the World , but no Aes Sedai were able to sense him during his time in Tar Valon. He gained much of his power battling the Fade throughout The Great Hunt . Padan Fain is also now a powerhouse in the series.
Sure, this is information we all know, and it’s not canon as it’s not in the books, but it was cool hearing people in the know say it.
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