Complete the following tasks to unlock the corresponding achievement. 
To view your achievements and stats in Steam, select "Community", then
"My profile", then "View all my games", then the game and view stats.

Achievement           How to unlock
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Accessory Restored  - The mayor of Formello lost something of great 
                      value. Recover it.
Acolyte             - Acolyte.
Acolyte             - Slay 100 foes.
Arch-Mage           - Learn 18 mage spells at level 3.
Captain of Avernum  - Complete all three game-winning quests in Avernum 
                      and never set the difficulty below Hard.
Cleric              - Learn 6 priest spells at level 3.
Crime and Atonement - Get a dread curse. Then cure it.
Dragonslayer!       - Avernum has five dragons. See if you can whittle 
                      that down a bit.
Gremlin's Gold      - Steal the treasure from Gremlin's Gold.
Halls Of Power      - Enter the Castle at last.
Herbal Adept        - Have ten potions made.
Herbal Master       - Have fifty potions made.
High Priest         - Learn 18 priest spells at level 3.
I Can Fly!          - Find something that lets you fly.
Lost Bahssikava     - Find the secret at the bottom of Lost Bahssikava.
Road To Rebellion   - Join the Scimitar.
Roads of Avernum    - Discover 10 locations of interest.
Safety For Avernum! - Destroy the demon Grah-Hoth, making Avernum safe 
                      from his menace at last.
Shovel Adept        - Dig up 5 caches.
Shovel Master       - Dig up 40 caches.
Slay 100 foes.      - Slay 100 foes.	
Student of Magery   - Learn a mage spell at level 3.
The Blade Reforged  - Reform a legendary blade from Avernum's past.
Valuable Ally       - Meet the Archmage Erika.
Ward of Avernum     - Complete all three game-winning quests in Avernum 
                      and never set the difficulty below Normal.
What's That Thing?  - Open a secret door.