Mysteries are a type of Card. They are the primary objective of the game; the Investigators win the game by solving Mysteries in order to quell the Ancient One's rising. For most Ancient Ones, investigators must solve 3 Mysteries before the Ancient One awakens in order to win. If the Ancient One awakens, the investigators will normally have to solve an additional final Mystery which is significantly harder in order to win the game.
Some Mysteries are specific to one space on the board, in which case a Mystery Token is used to indicate that the active Mystery is on a particular space.
Mysteries always resolve at the end of the Mythos phase, after which a new Mystery is drawn, or investigators win the game.
There are many different kinds of Mysteries, but each of them fits roughly into one of the following categories:
Each Ancient One will usually have a Mystery wherein the investigators need to solve Research Encounters in order to place Clues on the Mystery to solve it. These Research Encounters are specific to each Ancient One. When investigators have solved a sufficient amount of Research Encounters and placed the Clues gained from these encounters onto the Mystery card, the Mystery is solved at the end of the Mythos phase. This number is usually equal to the number of investigators (), or half that number. Any fractions are rounded up.
Defeat Epic Monster
Another common type of Mystery is one wherein investigators need to slay a spawned Epic Monster. The toughness of these Monsters generally scales with the . These "Mystery Monsters" also often have special requirements in order to damage them, such as the Hydra which requires the active investigator to possess the Sword of Y'ha Talla in order to damage it. These Monsters are generally very powerful and usually require the efforts of multiple investigators to take down.
Ancient Ones may also have a set of Special Encounter cards; some Mysteries require the investigators to encounter and successfully resolve a certain number of these cards in order to be solved. Special Encounters are generally more difficult than their Research Encounter counterparts, but have a wider variety of tests so that not only specialists will be the best to handle them.
Some Mysteries require the investigators to acquire a specific item (usually an Artifact). For example, Yig's Mystery, "Crown of the Serpent", requires the investigators to retrieve the Serpent Crown Artifact, perform a test, spend Clues, and then discard Ally Assets in order to solve the Mystery. In order to make these Mysteries fair, the Mystery often allows investigators to acquire the specific item they need if they would acquire an item from the specific item's category (i.e. if you would gain an Artifact, you may choose to gain the Serpent Crown instead).
Mysteries may sometimes spawn a number of Eldritch Tokens on the board. As encounters, investigators are usually required to perform various tasks successfully in order to place the Eldritch tokens on the Mystery card. Similar to the Research Encounter Mystery type, these Mysteries will often require a number of Eldritch tokens equal to or half the .
This type of encounter is the most varied. Some will require the discarding of items, while others may have the investigator Ambushed by Monsters. Typically, if the encounter is resolved successfully, the Eldritch token is discarded from the board. As with Research Encounters, any item that advances the mystery should not remove an Eldritch token from the board, but should use an unused token to mark the Mystery card.
When an Ancient One awakens, with the exception of Azathoth, investigators will need to solve a Final Mystery in addition to the regular Mysteries in order to win the game. These Final Mysteries will often involve either directly confronting the Ancient One as a Monster, or successfully resolving a certain number of Special Encounter cards. Unlike other Mysteries, the Final Mysteries are not normal cards; they are printed on the back of the Ancient One sheets in their top right corner.
Advancing the Mystery
Other than performing the task listed on the card, there are other ways to complete the Mystery. Some Assets like the Treasure Map, Tasks like Seek the Truth, or stages on an Adventure can "advance" the Mystery, making it easier to complete. How specifically the Mystery advances depends on the Mystery type.
If the Mystery is one that places Eldritch Tokens on a card or on the board, place an unused Eldritch Token on the Mystery. Do not remove any from the board.
If the Mystery requires spending Clues to place a Clue on a card, use an Eldritch Token to stand in for the Clue. If Clues are spent all at once to solve the Mystery, investigators will need to spend one less Clue in order to do so.
If the Mystery involves an Epic Monster, that Monster loses 2 Health. This health is placed on the Mystery card, so an Epic Monster like The Brood, which doesn't spawn immediately, still loses that 2 health once it appears. If there is more than one Epic Monster spawned, such as the Ancient Cthonian, than each of the monsters loses 1 Health.
Some extremely rare cards like the Promise of Power can completely solve a Mystery. If this happens, simply turn over the next Mystery at the end of the Mythos Phase. Remove the Epic Monster or any unused Eldritch Tokens when you do so.