Real simple. If i have no clues, can I play Forewarned?

No, I don't think so. You aren't able to place a clue on your location so the "then" part does not occur (see "Then" in the Rules Reference). Therefore playing the card will have no effect on the game state, which prevents you from playing it.

What if the Treachery is something like Twisted to His Will , and I want to remove "Forewarned" from my hand to get a higher chance to "randomly" discard weakness cards in my hand? Committing "Forewarned" to the test instead is undesirable, because to get rid of the weaknesses in my hand, I want to fail the test…

39 minutes ago, Assussanni said:

No, I don't think so. You aren't able to place a clue on your location so the "then" part does not occur (see "Then" in the Rules Reference). Therefore playing the card will have no effect on the game state, which prevents you from playing it.

Thanks, thats what I thought, though I didn't think to look up then to see why it was that way.

2 hours ago, Samea said:

What if the Treachery is something like Twisted to His Will , and I want to remove "Forewarned" from my hand to get a higher chance to "randomly" discard weakness cards in my hand? Committing "Forewarned" to the test instead is undesirable, because to get rid of the weaknesses in my hand, I want to fail the test…

"Desirable" or "undesirable" is irrelevant. The requirement is that it has to have the chance to change the game state. If you have no clues Forewarned can't do that, so you can't play it.

3 hours ago, Samea said:

What if the Treachery is something like Twisted to His Will , and I want to remove "Forewarned" from my hand to get a higher chance to "randomly" discard weakness cards in my hand? Committing "Forewarned" to the test instead is undesirable, because to get rid of the weaknesses in my hand, I want to fail the test…

hi @Samea , you can't randomly discard weaknesses from your hand. The only time (so far) a weakness can be discarded from your hand is if, on Terror from Beyond, 'event' is chosen and a player is holding Dark Memory. For things like Hidden cards in your hand, they cannot leave play (and cannot takes precedence over all else) except by the way described on the card. See also: Daisy's Necronomicon or Minh's King in Yellow weaknesses leaving play. So failing a Twisted to his Will test means you'll lose two of your cards at random, but not have a chance of sniping Hidden cards...

1 hour ago, zooeyglass said:

hi @Samea , you can't randomly discard weaknesses from your hand. The only time (so far) a weakness can be discarded from your hand is if, on Terror from Beyond, 'event' is chosen and a player is holding Dark Memory. For things like Hidden cards in your hand, they cannot leave play (and cannot takes precedence over all else) except by the way described on the card. See also: Daisy's Necronomicon or Minh's King in Yellow weaknesses leaving play. So failing a Twisted to his Will test means you'll lose two of your cards at random, but not have a chance of sniping Hidden cards...

This is incorrect.

You cannot choose to discard a weakness voluntarily, but if something else selects it for you (such as a random discard) that still works.

RR, page 21: A player may not optionally choose to discard a weakness card from hand, unless a card explicitly specifies otherwise.

If something forces you to discard a card at random, you're not choosing it, and this restriction doesn't apply. The clarification for Terror From Beyond exists because you're making a choice of card type which you know will result in the discard, so there was ambiguity over whether that counted as choosing to discard. It doesn't mean that it's the only way a weakness can leave your hand.

The "cannot leave play" text doesn't apply to discarding from your hand, because the cards aren't in play so the effect isn't active. Hidden is a completely different beast.

3 hours ago, Buhallin said:

"Desirable" or "undesirable" is irrelevant. The requirement is that it has to have the chance to change the game state. If you have no clues Forewarned can't do that, so you can't play it.

I agree that I cannot use the effect. But changing the number of my hand cards does not count as changing the game state? Especially in a situation where it changes the chance of getting rid of a weakness from "probably" to "guaranteed"?

RR, Page 21:

When a player wishes to initiate a triggered ability or play a card, that player first declares his or her intent. There are two preliminary confirmations that must be made before the process
of initiating an ability or playing a card may begin. These are:
=Check play restrictions: determine if the card can be played, or if the ability can be initiated, at this time. ( This includes verifying that the resolution of the effect has the potential to change the game state .) If the play restrictions are not met, abort this process.

The game state check is based on the resolution of the ability you're playing.

Ok, that makes it pretty clear. Hadn't found it, so thanks for pointing it out.