Shores of Idra
You will get this achievement after finishing the prologue and arriving at the shores of Idra at the start of chapter 1


The Liberator of Horryn
You will get this achievement after slaying the three Reaver leaders and liberating Horryn at the end of chapter 2
(also read below for the Puppeteer achievement)


Patricide
You will get this achievement after entering the temple through the secret tunnels and slaying "The Father" at the end of chapter 3


Gradla
You will get this achievement after acquiring a boat, sailing north of Idra, and arriving at Gradla during chapter 4


The Queen is Dead
You will get this achievement after slaying The Queen in the mines of Adler's Helm at the end of chapter 4


The Priory Opens
You will get this achievement after entering the namesake of this game at the start of chapter 5


The Eternity Gate
You will get this achievement after finding the buried secret within the Black Priory at the end of chapter 5


All is Darkness
Self-explanatory. Congratulation on finishing the game! Hope the ending rocked you.

Chitters-in-Darkness
Available chapter 1
To the north-east of the shores where you washed up is the wreck site of the Zephyr. The ship will be initially inaccessible.
You will find the surviving sailors at a camp by the coast. The leader of the survivor group will ask you to search for any remaining survivors. After you have rounded up the survivors, the leader will send some out to salvage the Zephyr, and ask you to check in on them. Wooden planks/ladders will have been set up, allowing you to enter the ship now. Enter, find what lies within, and the achievement is yours.


A Deeper Dark
Available chapter 1
Find the lighthouse keeper at the top of the lighthouse. He will ask you to find "the black blade" in honor of his keeper mate. Explore the caves underneath the lighthouse until you find an opening to some underwater tunnels. Enter, navigate through the underwater tunnels, acquire "the black blade" on the other side, and return to the lighthouse keeper with it to get this achievement.


A Light in the Darkness
Available chapter 1
After arriving at the refugee camp, the chamberlain will ask you to assist the refugees in two ways: food and medicine. Find the cook and cleric, and they will each give you a quest. Complete both of these quests to get this achievement. (Doing so will also make Iben the Ranger available to you as a companion)


Puppeteer
Available chapter 2
After entering Horryn, you will face the three leaders of the Reavers: Oleg, Kala, and The Butcher. Lucky for you, the three are in-fighting for power. Find the "sacred artifact" hidden underneath Horryn, and give it to The Butcher; he will ask you to ask Kala for help. Instead go to Oleg and inform him of The Butcher's betrayal, and then ask Kala to help Oleg. The three will kill each other, and you will get this achievement without having to even lift a finger.


Pilgrimage
Available chapter 4
After slaying The Father, the town of Horryn will regain a sense of peace, and the inn next to the northern gate will be open. There you will find a monk who will tell you about the five shrines scattered across the Outer Isles, and the proper prayer to activate these shrines. Simply follow the quest's directions and visit all five shrines, and the achievement is yours.


Such Wonders
Available chapter 4
You will get this achievement after arriving at Firgol - a short distance north of Idra, but still south of Gradla - and finishing its story. It is a very well written cosmic horror story featuring a very prominent cosmic horror figure, so I'll let you experience this for yourself ;)


Tower of Ash
Available chapter 4
Before you leave Horryn, you will be approached by an "assistant" of a great magi, asking you to help their master. Follow the quest's direction, arrive at the Tower of Ash, finish the quest within, and this achievement is yours.