Here is the full list of all 50 The First Templar - there are Steam Special Edition achievements.
Complete all levels on hard difficulty
Complete all levels on normal or higher difficulty
Complete the Arena
Complete all bonus objectives in the Arena
Complete the third Arena round without taking damage
Complete the second Arena round without taking damage
Complete the first Arena round without taking damage
Kill 15 opponents with Cyclone strike in the Arena
Find the Grail
Complete all levels on casual or higher difficulty
Complete all bonus objectives on all levels
Collect all Templar Chronicles
Find all collectible outfits in the game
Find all collectible weapons in the game
Explore the Temple of the Nine
Hit 4 enemies with your all-around attack at the same time
Hit 4 enemies with your charge attack at the same time
Survive the events of Friday the 13th
Successfully track 20 trails to their end
Perform 50 stealth kills
Heal your partner with the Mass Prayer skill
Kill an enemy grabbed by the other character
Defeat Muzafar
Win a duel
Kill 20 enemies with a ranged attack
Succesfully perform 10 counter moves
Set 50 objects on fire
Finish off 30 fallen enemies
Survive the siege of Acre
Acquire all Charge skills for Celian
A fleeing enemy is killed with a siege weapon by you or your partner
Kill an enemy by knocking him from a ledge
Distract and then stealth kill an enemy
Acquire all Dagger throw skills for Marie
Use the "Revive" action to help your teammate
Destroy a breakable wall
Advance a character to 5 stamina orbs
Advance a character to 5 health orbs
Kill all opposing squads with a siege weapon
Smash 5 enemy shields
Rescue Marie d'Ibelin
Discover a clue
Disarm a Bear trap
Defeat the beast
Save your partner from a dog attack
Complete all bonus objectives in a single level
Finish a whole level without both characters dropping below 1 health orb
Rescue the messenger of King Philip
Discover all chests in a single level
Kill 5 enemies in less than a minute without taking damage