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