Here is the full list of all 40 Dark Deity achievements.
Acquire all other achievements
Beat the game
Beat the game on Mortal mode
Beat the game on Hero mode
Beat the game on Deity mode
Have all stats increase on a single levelup
Chapter 27: Find the resolve to continue on
Chapter 25: Defeat one of your greatest enemies
Chapter 23: Resolve some long running familial tension
Chapter 20: Chase a dangerous foe away
Deploy 6 mounted units at once
Reach the max level of a single spell/weapon tree
Upgrade a unit to tier 3
Promote a character into the tier 3 corresponding to their tier 2
Chapter 16: Defeat a legendary creature
Start a chapter with aspects equipped to every deployed unit
Chapter 15: Complete the Trial of Soul
Have a unit take 150 damage and live in one chapter
Have a character move 150 tiles in a chapter
Take 25 or more turns to clear a chapter
Clear a chapter with Thae'Lanel, Corvan, or Maeve as your MVP
Clear Chapter 12 while the boat is below 30 health
Reach a max level bond
Chapter 12: Leave Etlan by way of sea
Chapter 11: Complete your most harrowing battle yet
Upgrade all of a unit's weapons to tier 4
Eat corn with Benji
Chapter 8: Have someone close to you witness a shameful act
Have a single unit dodge 20 or more times in one chapter
Defeat 15 enemies with one unit in a single chapter
Clear Chapter 7 while the aqueducts are over 50 health
Prove your worth to King Varic in under 8 turns
Chapter 5: Defeat the onslaught of undead
Upgrade a unit to tier 2
Upgrade your first spell/weapon
Chapter 1: Get drafted
Have one unit take 50% of the team's damage in a chapter
Take 0 damage from an attack.
Defeat an enemy in one hit.
Get a critical hit