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