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.
Collect all Steelface's Crates.
Complete all puzzle missions.
Defeat Mad Eye.
Defeat the Master Haunt.
Defeat the Necromancer.
Complete a non-tutorial mission without taking any damage.
Lock an enemy into a cell.
Defeat the Ice Demon.
Gain 20 power gems.
Defeat Oghma.
Collect a thousand gold pieces.
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.
Complete all missions on Hard without completing any missions with other difficulty levels.
Kill 9 enemies with a single action.