Here is the full list of all 20 Druidstone: The Secret of the Menhir Forest achievements.
Complete all main campaign missions.
Complete all missions with three stars.
Kill both Basilisks in The Pyriform Egg.
Complete a non-tutorial mission without using any spells.
Defeat the Master Haunt.
Complete a non-tutorial mission without any weapons equipped.
Gain 20 power gems.
Collect a thousand gold pieces.
Complete a non-tutorial mission without taking any damage.
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.