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