Instead of always having adventure cards face up, they are face down, adding a completely new unknown element to the game. It introduced a new game mode called 'Streets of Arkham' which completely changes the way the game is played. In February 2015, a second expansion called Gates of Arkham was released. The expansion also added many new cards to the game, including optional Master Mythos cards that can be used to increase the difficulty of the game.

The Cursed die also has the same sides as a green die but instead of using its result to aid you, if it matches the result of another die that was rolled, both dice are removed for the remainder of the adventure, making it harder to complete it. The Blessed die has the same sides as a green die and its result can be used to help complete adventure card tasks. Two new dice were added - a player uses the white die when they are Blessed and the black die when they are Cursed. It introduced the Blessed/Cursed mechanic. In June 2013, an expansion called Unseen Forces was released.

Giaco Furino of Fearnet wrote, 'If you're a Cthulhu fan this game is a must.' Michael Harrison of called it 'a fun, thrilling dice game'. This battle is designed to be extremely difficult with a low chance of success, so players must try to prevent the Ancient One from awakening at all costs. If the Ancient One awakens, the players must face it in battle. If the Ancient One is sealed, the players immediately win.

Rooms are explored until either the Ancient One is 'sealed' or 'awakened'.

If the player's investigator is devoured (either by losing all of their stamina and/or sanity), they lose what they originally had, a doom token is added to the doom track, and the player returns to play as a different investigator. Conversely, if they fail, they receive a penalty. If a player succeeds at completing all of the tasks in the room they are exploring, they obtain a reward.

Players take turns exploring a randomly generated room (there are Adventure cards and Other World cards).

Each player chooses an investigator to play as (usually randomly), each investigator having unique abilities.

There are also many other less powerful monsters that can appear during the game. Conversely, the Ancient One can be 'awakened' or released by a number of doom tokens. The Ancient One requires a certain number of elder sign tokens to 'seal' or imprison it.

Players randomly select a monster (known as an Ancient One) to oppose.

Elder sign vs arkham horror