Here is the full list of all 98 Stygian: Reign of the Old Ones achievements.
Arrive at the Black Pyramid alone
Attain hostile status against both factions (Mob and Cult)
Exterminate all Abenaki in Arkham
Kill 30 Bestial Enemies
Kill 20 Cultist
Talk with 5 spirits
Give the suicidal man a match
Lockpick 20 locked containers
Sacrifice Nameless Soldier to the Black Pyramid
Complete the game with Rational Belief System
Rest 30 times
Win the drinking contest with Carrion Jack
Suffer 30 Critical Failures
Unlock Game Over Screen: Greed Summons Doom
Obtain all of Crawford's personal belongings
Sacrifice Werner to the Black Pyramid
Complete the game with Nihilistic Belief System
Unlock Game Over Screen: Devoured in Nithon
Deal 30 Critical Attacks
Craft 10 different items
Kill 10 extra-terrestrial entities.
Fully unlock an entity on bestiary
Die 10 times
Use weakened poison on the mob men in the Distillery
Invest 10 points in the Science Skill
Invest 10 points in the Subterfuge Skill
Invest 10 points in the Medicine Skill
Invest 10 points in the Athletics Skill
Sacrifice Winston
Complete the game with Esoteric Belief System
Became Richter's model
Invest 10 points in the Occult Skill
Kill 25 Mob men
Acquire mental condition: Psychopathy
Create an Aristocrat character
Create a Criminal character
Create an Explorer character
Create an Academic character
Complete the game with Divine Belief System
Complete the game with Schizophrenia
Invest 10 points in the Survival Skill
Invest 10 points in the Stealth Skill
Acquire mental condition: Mania
Create a Soldier character
Invest 10 points in the Investigation Skill
Reach level 10
Invest 10 points in the Psychology Skill
Complete the game with Depraved Occultist Background
Complete the game with Materialistic Belief System
Pay Marino your so called debt
Invest 10 points in the Speechcraft Skill
Create an Occultist character
Learn All Spells
Collect 20 human skulls in a single playthrough
Unlock Game Over Screen: You Have Lost The Key
Acquire mental condition: Schizophrenia
Invest 10 points in the Melee Skill
Create an Performer character
Sacrifice the Man Lost in Nithon to the Black Pyramid
Sacrifice Outsider to the Black Pyramid
Unlock Game Over Screen: Trapped Eternally in Nithon
Complete the game
Complete the game with Humanistic Belief System
Complete the game with an Elder Character
Sacrifice Sonia Greene Carter to the Black Pyramid
Unlock Game Over Screen: Lobotomized
Unlock Game Over Screen: Fallen to the Stygian Questions
Enter 5 battles with a mask equipped
Unlock Game Over Screen: Devoured by the Street
Unlock Game Over Screen: Risen from the Dead
Acquire Mark of Cleansing: Friendship
Acquire Mark of Cleansing: Compassion
Acquire Mark of Cleansing: Love
Acquire Mark of Cleansing: Longing
Acquire Mark of Cleansing: Curiosity
Acquire Mark of Cleansing: Self-respect
Witness His majesty
Unlock Game Over Screen: Terrorized by a Daemon
Decipher the second line of the Dismal Man's Poem
Unlock Game Over Screen: Stabbed to Death
Summon the ghost in room 302 of Essex Hotel
Unlock Game Over Screen: You Have Gone Insane
Lose all your sanity
Decipher the first line of the Dismal Man's Poem
Unlock Game Over Screen: The Voyage of the Brain
Cast 30 occult spells
Invest 10 points in the Firearms Skill
Gain an ANGST Level
Bury the Widow
Read 10 documents
Give Cornelius' Watch to the Tunnel Kids
Unlock Game Over Screen : You Have Perished
Lie 5 times
Amass 1500 Cigs at a time
Become addicted to a substance
Acquire mental condition: Paranoia
Drive Hershel mad
Create an Investigator character