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