Here is the full list of all 100 OM6N achievements.
Kill 100 Insect demons
Kill 25 Insect demons
Kill 500 Insect demons
Kill 1000 Insect demons
Kill 25 Fire demons
Kill 50 Fire demons
Kill 500 Fire demons
Kill 1000 Fire demons
Kill 25 Cultist demons
Kill 50 Cultist demons
Kill 500 Cultist demons
Kill 1000 Cultist demons
Kill 25 Disturbed demons
Kill 50 Disturbed demons
Kill 500 Disturbed demons
Kill 1000 Disturbed demons
Kill 25 Hound demons
Kill 50 Hound demons
Kill 500 Hound demons
Kill 1000 Hound demons
Kill 25 Brute demons
Kill 50 Brute demons
Kill 250 Brute demons
Kill 500 Brute demons
Kill 25 Horror demons
Kill 50 Horror demons
Kill 100 Horror demons
Kill 250 Horror demons
Complete chapter I
Continued into the darkness..
Encountered demonic forces..
Angered the force of Hell..
Slayed one of Meric's dark lords..
Survived your first night
Survived for 7 nights
Survived for 14 nights
Survived for 20 nights
Survived for 25 nights
Survived for 31 nights
Complete Chapter XII
Complete Chapter XIII
Complete Chapter XIV
Complete Chapter XV
Complete Chapter XVI
Complete Chapter XVII
Complete Chapter XVIII
Complete the Tutorial
Kill 25 Stalkers
Kill 100 Stalkers
Kill 500 Stalkers
Kill 1000 Stalkers
Kill 10,000 Stalkers
Kill 25 Death Souls
Kill 100 Death Souls
Kill 500 Death Souls
Kill 1000 Death Souls
Kill 10,000 Death Souls
Kill 25 Tech Souls
Kill 100 Tech Souls
Kill 500 Tech Souls
Kill 1000 Tech Souls
Kill 10,000 Tech Souls
Kill 25 Grunts
Kill 100 Grunts
Kill 500 Grunts
Kill 1000 Grunts
Kill 10,000 Grunts
Kill 25 Pit Beasts
Kill 100 Pit Beasts
Kill 500 Pit Beasts
Kill 1000 Pit Beasts
Kill 10,000 Pit Beasts
Kill 25 Defilers
Kill 100 Defilers
Kill 500 Defilers
Kill 1000 Defilers
Kill 10,000 Defilers
Kill 25 Soul Harvesters
Kill 100 Soul Harvesters
Kill 500 Soul Harvesters
Kill 1000 Soul Harvesters
Kill 10,000 Soul Harvesters
Kill 25 Doom Knights
Kill 100 Doom Knights
Kill 500 Doom Knights
Kill 1000 Doom Knights
Kill 10,000 Doom Knights
Kill 10,000 Zombies
Kill 10,000 Hounds
Kill 10,000 Forgotten Souls
Kill 10,000 Suicide Souls
Kill 10,000 Imps
Kill 10,000 Butchers
Kill 10,000 Brutes
Reach Rogue Rank 15
Reach Rogue Rank 30
Reach Rogue Rank 50
Reach Rogue Rank 100
Complete a total of 10 Omen portals
Find a total of 10 Secrets