The Event Horizon expansion provides a new way to play Catacombs Conquest. The Sword and Skull team card decks from the base game are replaced with the decks from this expansion. Each card has two shot sequences: a primary shot sequence that can be played immediately and a secondary shot sequence that must be activated by matching a "star sign" symbol. Each player has a set of five Leader cards that start the game face-up in front of them. Players then build their deck by choosing two sets of cards to play with from their team (fighters, spell casters, hunters, creatures, or explorers).

Players must play a card from their hand (or flip over one of their Leader cards) to perform the card's primary sequence and may choose to play another card to activate the primary card's secondary shot sequence (a Leader card can also be used for this purpose). Anytime a player cannot draw a card, their discarded cards are reshuffled.

The expansion adds the leech shot and new rules for the fireball, missile, and wave ranged shots. It also introduces the new "Dot" connector which enables *both* Character discs to perform a shot. The game features some new characters including the Dwarven Warrior, Siren, Oozing Maw, Stone Sentinel and Battle Robot.