Uses (4 secrets). If Forbidden Knowledge has no secrets, discard it.
Exhaust Forbidden Knowledge and take 1 horror: Move 1 secret from Forbidden Knowledge to your resource pool, as a resource.
Forbidden Knowledge is the definition of Risk-Reward that makes Mystic a tricky faction. 0 cost for 4 resources is great, 4 Horror is not so great. Agnes Baker can use this to her advantage by dealing damage when she receives Horror, but you are limited to once per Phase. During your investigation phase you can take a free resource, take a horror, and deal a damage. If you're in a really tight spot where you need to eliminate an enemy, you have a player window during the Mythos phase after encounter cards are drawn and immediately before enemies attack. That's a total of 3 damage dealt, 3 resources gained, but also 3 horror taken in a single round, without having to make a skill check.
This is a useful asset, but you should only use it when absolutely necessary, or you'll find yourself dropping out of scenarios from sanity loss. Unless you're with friends that can patch you up with First Aid, you're going to be saving your second copy for the Intellect icon, or replacing it with leveled up cards as a campaign progresses.
To pass on this gem with Carolyn Fern would just be Foolishness. This is so good on Carolyn with Peter Sylvestre in play. 2 resources every turn for 4 turns -- awesome. With Dr. Milan Christopher, this setup, Liquid Courage, and Teamwork, The Nerd Girl becomes killer support, and the bigger the group, the better. Before you know it, everyone's rolling in it. Have her hand a Dark Horsed Silas Marsh a St. Hubert's Key while you're passing out goodies, and they may as well start calling you Dr. Frankenstein, because you've just created a monster.