The Crystal Warriors (クリスタルの戦士, Kurisutaru no senshi?), also known as the Ark Angels, are a group of five beings created by the Zilart. While seemingly anachronistic, since the Zilart exist before the five enlightened races, the Crystal Warriors are formed from the same stuff that formed into the five enlightened races, thus explaining their similarities.
In the beginning of the Rise of the Zilart chapter of Final Fantasy XI's plot, the player confronts Kam'lanaut and Eald'narche after defeating the Shadow Lord. The two Zilart brothers summon the "Crystal Warriors" to assist them in their plans, namely getting rid of the player and the game's NPC heroes (Zeid, Lion, and Aldo).
While Raogrimm slows down and distracts them, the heroes (and the player) escape with their lives. Later in the plot, when the player gains access to the region of Tu'Lia, the Ark Angels can be fought one at a time in separate boss battles. The Ark Angels can also be fought in a separate boss battle with all five together with a separate quest from the original mission.
The player must defeat any one of the Crystal Warriors for Zilart Mission 14: Ark Angels.
Alternatively, the player can engage all five at once for the quest Divine Might, which also counts towards completing the mission.