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.
Kill both Basilisks in The Pyriform Egg.
Complete all missions with three stars.
Complete all missions on any difficulty level.
Complete all main campaign missions.
Collect all Steelface's Crates.
Complete all puzzle missions.
Defeat the Necromancer.
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.
Complete a non-tutorial mission without using any spells.
Defeat the Master Haunt.
Complete a non-tutorial mission without any weapons equipped.
Complete The Oakshield's Demand under 90 seconds.
Lock an enemy into a cell.
Kill 9 enemies with a single action.