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 all main campaign missions.
Complete all missions on any difficulty level.
Collect all Steelface's Crates.
Complete all puzzle missions.
Defeat Mad Eye.
Complete a non-tutorial mission without taking any damage.
Defeat the Necromancer.
Defeat the Ice Demon.
Gain 20 power gems.
Collect a thousand gold pieces.
Defeat Oghma.
Complete all missions with three stars.
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.
Complete all missions on Hard without completing any missions with other difficulty levels.
Kill 9 enemies with a single action.