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