Here is the full list of all 21 Betrayer achievements. It takes around 12-15 hours to unlock all of the achievements on Windows.
Stealth-kill 200 enemies
Find all of the lost souls
Kill 1000 enemies
Kill 100 enemies with tomahawks
Stealth-kill 20 enemies with melee attacks
Find all graves
Free all of the lost souls
Free six lost souls
Unlock all destinations
Kill 3 Thieves
Acquire all unique weapons
Find all treasure chests
Find all page scraps
Find all clues
Discover the name of the Maiden in Red
Find 6 lost souls
Unlock 10 destinations
Find 15 graves
Find 25 clues
Find 20 page scraps
Find 40 treasures chests