Here is the full list of all 15 We are the Plague Demo achievements.
Defeat any enemy
Take with you all the lost souls
Collect and apply your first tattoo
Leave the Fortress of Vexation
Collect and apply your first mark on the weapon
Like master, like man
Spare the monster
Kill the monster
Kill all enemies
Visit all locations
Win all battles without any casualties on your side
Explore every location with 100%
Finish the prologue
Finish the prologue on 'Plague Priest' difficulty... Good luck
Unlock all achivements