Here is the full list of all 20 Druidstone: The Secret of the Menhir Forest achievements.
Complete all missions with three stars.
Complete all missions on Hard without completing any missions with other difficulty levels.
Complete all missions on any difficulty level.
Complete all main campaign missions.
Kill 9 enemies with a single action.
Complete a non-tutorial mission without taking any damage.
Complete a non-tutorial mission without any weapons equipped.
Complete a non-tutorial mission without using any spells.
Complete The Oakshield's Demand under 90 seconds.
Complete all puzzle missions.
Collect all Steelface's Crates.
Defeat the Master Haunt.
Defeat the Necromancer.
Defeat Mad Eye.
Defeat the Ice Demon.
Gain 20 power gems.
Lock an enemy into a cell.
Defeat Oghma.
Collect a thousand gold pieces.
Kill both Basilisks in The Pyriform Egg.