Upon starting a run, you'll have to select which Old God you're trying to prevent the awakening of. This is represented by the Doom counter, which steadily increases over the course of a run, and the game is over if it reaches 100%. These eldritch beings affect which events you'll run into, and each have their own Old God's Rule that affects gameplay.
Cthac-Atorasu, the Spider God[edit]
"The terrible spider god, Cthac-Atorasu, is once again awakening from its slumber...
Blinded by its impossible to satisfy hunger, it has started to tear down the walls of its cocoon prison.
Stop it before it devours us all."
Unlocked by: Available from the start.
Old God's Rule: Eternal Web - Prevents fleeing from enemies.
Events: Bitten, Small Cocoon
Ath-Yolazsth, the Towering Eye[edit]
"Media reports on various people with seemingly supernatural skills.
Could it have any connection with the news about stars disappearing from the night sky?"
Unlocked by: Available from the start.
Old God's Rule: Cosmic Gift - Changes all spells' Reason costs into Doom costs. Players also begin knowing a random spell (this stacks with Kirie Minami's random spell, giving her two).
Events: Stargazing, Towering Eye's Prophet
Goizo, the Thing Forsaken by God[edit]
"Banished to the other dimension a long time ago, Goizo has found a way to use mirrors to ensnare and teleport its prey straight through the glass.
Glimpses of horror lurking behind your reflection are just the beginning..."
Unlocked by: Available from the start.
Old God's Rule: Mirror Horror - Increases Doom penalty from resting at home.
Events: Agents of Goizo, Desperate Solution, Treacherous Reflection
Ithotu, the Devouring Fire[edit]
"The supposed cause of the destruction of the library of Alexandria, a perfect black marble statue has been recently discovered and is currently waiting in the museum of our town for its premiere..."
Unlocked by: Available from the start.
Old God's Rule: Entropy - Increases damage both taken and dealt.
Events: Scorching Sun, Pillar of Flame, Upcoming Exhibition
Ygothaeg, the Irresistible Gaze[edit]
"The nuclear tests in the Pacific region have awakened something evil in the depths of the Mariana Trench. Ygothaeg is now slowly emerging, ready to once again rule the coastlines."
Unlocked by: Use the Glass Eye item and the Third Eye spell in the same run.
Old God's Rule: Gaze of Ygothaeg - Incurs a 2% Doom penalty if you investigate a location Ygothaeg doesn't want you to (displayed over the sidequest card).
Events: Irresistible Clouds, People Under the Waves, Break the Gaze
Herald of the Shattered Court[edit]
"It iSs mYy pleeaSurRe to invvvite evERyoNe tto tHhe mosTt magnnifFicent BALL of yyourR livVves! AnNd thHe laSst onNe."
Unlocked by: As Moriko Ishii, defeat an enemy with the Chainsaw weapon.
Old God's Rule: Court's Calling - Causes an additional town status to occur upon finishing a mystery.
Events: The Trumpets of Shiokawa, Cheerleading Routine
Ktu-Rufu, the Dreaming[edit]
"The gargantuan body of Ktu-Rufu lies submerged within a sunken city, where its boundless reveries sow turmoil and insanity. When the stars are right, Ktu-Rufu will awaken once more and the nightmare shall begin..."
Unlocked by: Encounter the Dream Salesman, Dream Eater, and Dream Devourer enemies in one run.
Old God's Rule: Nightmares - Whenever Reason is lost, the Doom counter rises as well. Players also have the "Just a Dream?" status effect, which lowers Doom by an additional 2% upon finishing a mystery.
Events: Bad Dreams, Crumbling Reality
Zhectast, the Horror from the Stars[edit]
"A big meteorite fell down on a field in a village near Shiokawa. A farmer who found it contacted researchers from Tokyo, but when they arrived, all they found was a wet puddle of pinkish goo."
Unlocked by: Complete seven runs.
Old God's Rule: Perfect Form - Reduces all stats by -1 for every 21% of the Doom counter.
Events: The Perfect Form (A), The Perfect Form (B), The Perfect Form (C), The Perfect Form (D)
Eh-Żhal, the Inconceivable Sadness[edit]
"Slithering through the twisted tapestry of dimensions, the festering servants of Eh-Żhal latch onto new vessels, ravenously devouring the very essence of human suffering, swelling their grotesque forms. Slowly but inevitably, the hosts succumb, their bodies and minds becoming one with the burgeoning god."
Unlocked by: Complete a run as Yumiko Ichimura.
Old God's Rule: Al-Misery - Players have the "Misery Parasite" status effect, which raises Doom at the end of a mystery depending on semi-randomized conditions (such as how many spells the player knows.)
Events: N/A