Here is the full list of all 20 Druidstone: The Secret of the Menhir Forest achievements.
Lock an enemy into a cell.
Kill both Basilisks in The Pyriform Egg.
Complete a non-tutorial mission without using any spells.
Complete a non-tutorial mission without any weapons equipped.
Defeat the Ice Demon.
Gain 20 power gems.
Collect a thousand gold pieces.
Defeat Oghma.
Complete all main campaign missions.
Complete all missions with three stars.
Defeat the Master Haunt.
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.
Defeat Mad Eye.
Defeat the Necromancer.
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.