Overview
Mythos cards are a primary method through which the game will cause the investigators to lose. The third phase of every round is called the Mythos Phase, where a Mythos card is drawn from the Mythos deck. After resolving its effects, play continues to a new round. There are three types of Mythos cards with three different difficulties.
Mythos Deck
At the start of the game, players must build the Mythos Deck from the 217 Mythos cards available (as of Masks of Nyarlathotep). The Mythos Deck is built according to the composition of Green, Yellow, and Blue Mythos cards as indicated on the Ancient One sheet being played.
Every Mythos deck is split into three separate Phases. Each Phase is created and shuffled independently of another, and then put back together with the third Phase on the bottom, the second Phase in the middle, and the first Phase on the top.
Before constructing the Mythos Deck, modify the difficulty of the deck as you wish. Then, separate the Green, Yellow, and Blue cards into three separate decks and shuffle each deck.
For each Phase, check the Ancient One sheet; draw a number of colored Mythos cards as specified by the Phase that you are constructing, and then shuffle these cards together, then set them aside. It is recommended that you construct the deck from Phase 3 to Phase 1 so that you can simply stack the Phases on each other when you are done constructing each.
Finally, recombine the Phases, and set aside the remaining Mythos cards; those cards will not be used for the current game.
Triggering a Mythos Card
When a Mythos card is drawn, players should resolve all of its icon effects from left to right as they are listed at the top bar of the Mythos card. After all icons effects have been resolved, the Mythos card's main effect should be resolved.
Mythos Card Icons
| Icon Name | Symbol | Effect |
|---|---|---|
| Advance Omen | 
|
Move the Omen Tracker Token clockwise 1 symbol, then advance Doom accordingly |
| Reckoning | 
|
Resolve  effects in the following order: Monsters, Ancient One, Mythos Cards, Investigators
|
| Spawn Gates | 
|
Spawn a number of Gates and Monsters according to the Reference card being used |
| Monster Surge | 
|
Spawn X Monsters on each Gate corresponding to the current Omen, where X is noted by the Reference card being used. If no Gates are present that correspond to the current Omen, spawn 1 Gate instead |
| Spawn Clues | 
|
Draw and spawn Clues according to the Reference card being used |
| Place Eldritch Tokens | 
|
Place a number of Eldritch Tokens as listed on the symbol on the Mythos Card that triggered this effect |
| Spawn Rumor | 
|
Place a Rumor token on the space indicated on this symbol |
Deck Depletion
If a Mythos Card needs to be drawn, but the Mythos Deck is empty, the Mythos Phase ends immediately. This will possibly trigger the solving of a final Mystery, in which case the investigators win the game. However, if investigators have not won by this point, they automatically lose the game.
Mythos Card Types
Example of a Green, Hard Mythos Card
Green Mythos Cards
Green Mythos Cards are generally the second most common Mythos card; they will always cause the Omen to advance, trigger a Monster Surge, and spawn Clues. The Monster Surge and spawning of Clues is determined by the Reference card being used.
Green Mythos cards will usually have an immediate event, although some will have an ongoing effect.
As of Masks of Nyarlathotep there are 78 Green Mythos Cards: 20 Easy, 37 Normal, 21 Hard.
Example of a Yellow, Easy Mythos Card
Yellow Mythos Cards
Yellow Mythos Cards are generally the most common Mythos card; they will always cause the Omen to advance, trigger a , and spawn Gates. The spawning of Gates is determined by the Reference card being used.
Yellow Mythos cards always trigger an immediate event.
As of Masks of Nyarlathotep there are 92 Yellow Mythos Cards: 22 Easy, 41 Normal, 29 Hard.
Example of a Blue, Normal Mythos Card
Blue Mythos Cards
Blue Mythos Cards are the least common Mythos card; they will always spawn Clues.
Blue Mythos cards always initiate a Rumor. Often, Eldritch Tokens will be placed on the card in order to keep track of when the Rumor's fail effect triggers; the starting number of Eldritch Tokens, should they be required, is printed on the symbol on the top bar of the Mythos card.
As of Masks of Nyarlathotep there are 47 Blue Mythos Cards: 11 Easy, 19 Normal, 17 Hard.
Mythos Card Difficulties
Mythos cards are also differentiated by difficulty. Difficult Mythos cards are indicated by a red tentacle border around the Mythos card's name, while easy Mythos cards are indicated by a blue frost border around the Mythos card's name. Mythos cards without a border around their name are "normal" difficulty.
If the players wish to make the game easier or harder, they may build the Mythos deck using cards only from one or more of these difficulty categories in order to modulate the difficulty of the game. For example, removing all difficult Mythos cards creates a Mythos deck consisting of only normal and easy difficulty Mythos cards, which will make the game easier to win.
