Here is the full list of all 21 Praetorians - there are HD Remaster achievements.
Complete all four Tutorial Missions
Complete the mission Crossing the River Arar
Complete the mission Escort to Bibracte
Complete the mission Of All the Gallic Tribes
Complete the mission Divide and Conquer
Complete the mission The Everlasting Frontier
Complete the mission A Land Lost in the Mist
Complete the mission Greed
Complete the mission Fear the Eagles
Complete the mission Cold Treason
Complete the mission When All Hell Breaks Loose
Complete the mission He Who Dares…
Complete the mission Homecoming
Complete the mission War of Attrition
Complete the mission Alea Iacta Est
Complete the mission War Within the Mountains
Complete the mission Let the Hunt Begin
Complete the mission The End of the Republic
Complete the mission The Battle for Alexandria
Complete the mission Friends and Allies
Complete the mission Just One… More… Fight