You must either play this card (spend an action and pay 2 resources to place 1 doom on the agenda) and place it into the discard pile, or take 2 horror at the end of each turn while it's still in your hand.
You cannot choose to discard a Weakness card from your hand, neither for exceeding the max number of cards in hand, nor for card effects that make you 'choose and discard'. You can, however, discard it at random, or if an effect makes you discard your whole hand.
- If you have Dark Memory in your hand when you draw Amnesia, you must choose and discard all other cards, and keep Dark Memory.
Place 1 doom on the current agenda. This effect may cause the current agenda to advance.
Forced - If Dark Memory is in your hand at the end of your turn, reveal it and take 2 horror.
A review of a Weakness? Yep. Inasmuch as Weaknesses go, I "like" Dark Memory. It is unique among investigator Weaknesses (so far) in that it can actually have beneficial effects in addition to its harmful ones. [Possibly Baron Samedi and Smite the Wicked could be considered to have positive side-effects, but not quite as tangible as Dark Memory.] One of the features I like most about Arkham Horror LCG is that magic is frequently a double-edged sword. Positive effects often have negative blowback--as with Ward of Protection, Shrivelling, Blinding Light, Forbidden Knowledge, etc.--and negative effects can occasionally have a positive side-effect, as with Agnes Baker's innate effects.
Dark Memory falls into the latter category. While its negative effects--spend an action to play it, pay 2 resources, and gain 1 doom token OR keep it in your hand (taking up space toward your hand maximum) and take 2 horror at the end of each turn--are bad, both can have a silver lining for Agnes. In the first case, as a Spell, when played, Agnes can draw a card with Heirloom of Hyperborea. While this doesn't mitigate its ill effects, at least it's something positive, and Dark Memory is discarded. In the second case, Agnes can take one of those two horror on herself to deal 1 damage to an enemy at her location. Since her horror-for-damage effect is limited to once per phase and this horror is taken in the Upkeep Phase, this enables her to potentially deal damage with her effect in all 4 Phases. Yes, that's a lot of horror for Agnes to take; but, then, she's probably prepared for it with assets that soak horror, and healing from Fearless and/or Clarity of Mind.
All in all, if an investigator is going to have a weakness, it's nice if that weakness can also provide some kind of benefit as well. All the more reason Agnes Baker is one of my favorite investigators.