Because the dimensional barriers near Kingsport are so thin, the area is plagued with dimensional rifts. These are mobile tears in the fabric of the universe from which monsters emerge if they are left unattended. At the start of the game, the three rifts are closed, and the corresponding rift markers are placed randomly on the three Rift Tracks on the Kingsport board to show this. Each Rift Track consists of four spaces divided into two groups, with each group having a different monster movement pattern (as seen on the mythos cards) associated with it.
Rift Track Mechanics
Each time a Mythos card is resolved, check to see if the monster movement pattern on it matches any of the patterns next to a closed rift. If it does, draw a rift progress marker from the pile at random, turn it faceup, and place it in one of the two spaces next to the monster movement pattern that was drawn. If both spaces next to the monster movement pattern are already full, do not draw another rift progress marker.
If, after placing a new rift progress marker, all four spaces next to a closed rift now contain rift progress markers, the rift opens and is placed at the gate location shown on the Mythos card that was resolved this phase even if there is an elder sign token there).
Open rifts activate during the Mythos phase starting on the turn after they open. Each rift is marked with a dimensional symbol in either white or black (for example, a black moon). If a drawn Mythos card indicates that monsters with the rift’s dimensional symbol move, the rift first moves like a normal black-bordered monster along the indicated arrow, and then a monster is drawn from the monster cup and placed in the rift’s new location. So, for example, if the mythos card indicates that monsters with the moon dimensional symbol should move along white arrows, the black moon rift would move along the white arrow from its current location, and then a monster would be placed at the rift’s new location. Monsters entering the board through rifts are still subject to the usual monster limit.
In addition, if a rift moves along an arrow of the same color as its dimensional symbol, a doom token is added to the Ancient One’s doom track. So, to use the black moon rift again as an example, if a Mythos card indicates that monsters with the moon dimensional symbol should move along black arrows, the black moon rift moves along the black arrow from its current location, spawns a monster in its new location, and a doom token is added to the doom track. Clearly, open rifts represent a grave danger to Arkham.
Investigating and Closing Rifts
Each rift progress marker depicts a location in Kingsport where information about that rift may be found. If an investigator has an encounter at the depicted location, he has investigated that rift progress marker. If the rift associated with a rift progress marker is closed when a character investigates it, the rift progress marker is discarded back into the pile of rift progress markers, to be used again later. If the rift associated with the rift progress marker is open when a character investigates it, however, the rift progress marker is turned facedown. Once all four rift progress markers associated with an open rift have been turned facedown, the rift closes; the rift marker is returned to its Rift Track and the four rift progress markers are discarded to their pile.
The monster movement symbols and colors on the drawn Mythos card are what determines whether a rift spawns a monster or adds a doom token, NOT whether or not the rift actually moves. Thus, a rift that has moved into a vortex will no longer move, but will continue to add doom tokens and spawn monsters directly into the vortex.