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