When Keeper is playing mythos card, must he show it to the investigators? Some mythos require attribute check. For example Locked Behind when failed causes some negative effect. My question is: shall I know what is at stake, to perhaps decide about spending skill to add Luck to tested attribute?

Cr33z said:

Page 25 of the rulebook:

Mythos and Trauma Cards: Any cards in the keeper’s hand are secret and may not be seen by the investigators. Once played, these cards are displayed faceup for all players to see. The only exception are the few Mythos cards which are specifically played facedown. Only the investigator who receives one of these Mythos cards may examine it, and it may not be looked at by other investigators (but may be examined by the keeper).

And just because it's effectively the same:

Combat Cards are shown to everyone too before the investigator decides to use a Skill Point or not.
Forgot that the last time I played, might've motivated some players to spend their points (especially on "you hit, deal damage + X additional damage" or stuns, etc.)