Dunwich Horror adds 8 Investigators, 4 Ancient Ones, 15 Item cards, 25 Unique Item cards, 21 Spell cards, 11 Skill cards, 5 Ally cards, 4 Condition cards, 28 Monster tokens, 36 Mythos cards and 32 Gate cards to the base set. It also adds new location cards, for a total of 14 cards at each location instead of 7. Dunwich Horror also adds many new mechanics. Players can take a train to Dunwich (a new board that is placed on the end of the Arkham board), which adds 9 new locations and 2 new outer worlds to the game. Players reduced to 0 stamina or sanity have the option of drawing from the Injury or Madness deck, acquiring permanent handicaps, rather than losing half their items and clue tokens. Gates are no longer permanently sealed (a popular house rule in the base game). Condition cards provide benefits to all players may be activated. And, of course, the Dunwich Horror itself - not as strong as the Ancient Ones, yet far stronger than any other monster in the game.

This expansion also features revised rules and an FAQ that addresses many of the perceived faults of the base game. Replacement cards are provided for items and spells that are subject to errata.