Marathon Horror is a variant of Arkham Horror in which many Ancient Ones are threatening the world all in quick succession.
Marathon Horror uses the “weak seals” variant (courtesy of Dail):
If you would place an elder sign token on the board, instead place 3. If a gate tries to open at a sealed location, instead remove an elder sign token. If a gate burst occurs at a sealed location, remove 3 tokens. If there is still at least one sign on the location, the gate burst is prevented. These tokens don’t individually count as elder signs; you need 5 sealed locations, not 5 elder sign tokens on the board, to win through seals.
When setting up the game, choose an Ancient One at random. Choose a Guardian (if desired) as normal. Choose a Herald at random. Finish setting up normally.
If the investigators manage to get 5 seals on the board, the Ancient One is not defeated. Instead, this is rewarded by a choice: Choose a new Guardian to help you (in addition to the current Guardian), or remove 5 doom tokens from the doom track. If the number of seals falls below 5 and comes back up, you do not get the reward again.
If the investigators manage to get a closed gates victory, the current Ancient One is defeated. Discard it and all gate trophies, and as a reward, the investigators may either discard one of the current Heralds as well, or choose a new Guardian to help (in addition to the current Guardian).
If the Ancient One awakens, one of the Guardians flees. The investigators must choose a Guardian to discard. If the Ancient One is defeated in combat, the players may choose to close a Gate as a reward. All Heralds remain in play to help the next Ancient One.
If the Ancient One is defeated (through a closed gates victory or through combat), the investigators get one free turn. At the end of that turn, before the Mythos Phase, choose a new Ancient One and a new Herald, both at random.
When choosing a new Guardian to help, you may choose Guardians that have previously fled. When choosing a new Herald at random, do not include defeated Heralds. If there are no Heralds left, the new Ancient One instead begins with a doom token on its doom track for each time this has happened. When choosing a new Ancient One, do not include defeated Ancient Ones. If there are no Ancient Ones left, the investigators win!