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