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