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