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