The ones you can obtain simply by following the story. Some are choice-dependent, but you should always get one or the other.

Oathbreaker

After the opening sequence, state you've chosen the adult demonspawn as your ally.




Mother

After the opening sequence, state you've chosen the younger demonspawn as your ally.




Outlaw Allies

Sir Brash has joined the party.




Infidel

Sabotage one of the Cult of Ravage's quest. You can do so by following Sinallion's advice and pay the thugs for the last task, or offer the Church's food to Sinallion himself instead of giving it to someone who needs it.




True Sister

Follow the priestess' requests to the letter: feed the poor and kill the thugs instead of paying them.




Infernal Allies

Rescue your demonspawn ally.




Sea Dogs

Board Sinallion's ship.




Infernal Traveler I

Travel to Raze's lair.




Justice

Free the trapped demonspawn in Raze's lair.




Infernal Traveler II

Travel to Thayn's lair.




Hero

Defy Thayn and side with humanity.




Omen

Side with Thayn and agree to become the Apocalypse's mother.

Tied to the romance options you choose. Starting a romance from the opening sequence allows you to kiss that character as soon as you see them, thus earning the related achievement immediately.

Witch Love

Kiss Ari.




Knight Love

Kiss Sir Brash.




Bard Love

Sinallion is a new character, and cannot be selected right from the get-go. You get two chances to kiss him: the first in Bekka's inn if you choose to visit him after he's been wounded and hesitate to leave him like that, the second in the ship's cabin.




Demonspawn Love

Kiss Raze.




Demon's Bride

To earn this achievement, you need to be careful, because it can only trigger if Thayn visits you in the bed in Raze's lair. You must start the game selecting "no one" when you're asked who you're having an affair with and then proceed to avoid Ari, Brash and Raze's attempts to flirt with you. You can, however, kiss Sinallion, as he doesn't join you in bed, unlike the other love interests.

You select your alignment right in the beginning, so you can start from 80% of your chosen alignment (i.e. 80% Good 80% Tactful). The achievements can be easily earned if you act accordingly to your choices. On an estimate, each relevant dialogue option earns you around a 5% shift.

Do-Gooder

Reach 100% Good. Be nice, don't be eager to murder, spare whoever you can and criticize Brash and Ari's tendency to murder when you get the chance.




Wicked

Reach 100% Evil. Look forward to all the murder and mayhem you can spread, don't hesitate to stab certain people, plot with Ari, Brash and Raze.




Don't Talk To Me

Reach 100% Defiant. Be rude, if you see a dialogue option containing a swear word, smash it. I recommend starting a new save solely to insult everyone and get this achievement, because otherwise you risk getting stabbed in the back by your very outraged companions during Thayn's confrontation.




Nice and Smiling

Reach 100% Tactful. Be polite. At least outwardly.

These achievements can be earned during the story, but they require you to take some extra steps.

Assassin I

Earn the Poison I skill. To do so, you need 100 Poison Powders.




Assassin II
Earn the Poison II skill, at the cost of 150 Poison Powders.




Assassin III

Earn the Poison III skill, at the cost of 200 Poison Powders.




Town Healer

Sell 30 Healing Salves. You need a total of 300 Healing Powders to do so, but you can also find some salves by looting corpses or barrels. Any merchant is fine. IMPORTANT UPDATE: can actually be achieved through multiple playthroughs.


Please note that, to craft powders, you need to consistently look for ingredients to use. Flowers, mushrooms and animal/monster parts all qualify: for example, a Big Spider Leg can be ground into 2 Poison Powders, just like the Glowing Mushroom, while Deer Antlers and Poppies can be transformed into Healing Powders. Flowers and mushrooms have a high chance of respawning when you visit the area again. An especially good spot to farm Poison Powder ingredients is the unded-infested area in Ravage. The undead respawn frequently every time you visit it, and they have a chance to drop from 1 to 2 Undead Eye, which can be used to get 2 Poison Powders each.



Defiler

Sell a human skull. Skulls are usually looted from drawers, armoires and Crawlers (the slugs you find in Inferno). You should come across one of the first skulls in Raze's lair.




Demoncat

Find Mr. Edgardo. He's in Raze's lair. Interact with him to get the achievement.

A warm thank you to the Demonheart's discord community, for loving the game as much as I do, especially to Ash and gabu, who have both provided for priceless information concerning the Demon's Bride achievement and saved me a headache. <3