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