Here is the full list of all 20 ARENA an Age of Barbarians story achievements.
Defeat the sorcerer Ghoor!
Kill that damn goblin!
Do a Special Move 30 times!
Kill Kan-Dread the Destroyer!
Perform a 10-hit combo with any enemy!
Win a fight without getting hit!
Win a fight without parry!
Perform a 20-hit combo with any enemy!
Dismember 100 enemies!
Perform 30 Counterattacks!
Kill the giant serpent!
Rescue Xodan from the torture chamber!
End the game as Rahaan
End the game as Sion
End the game as Mombata
End the game as Sheyna
End the game as Ayla
End the game as Xula
End the game as the Deathbringer
End the game as She-Red