Castillian is a mysterious Forsaken mage serving the Argent Dawn and a character in the Warcraft franchise.
The undead mage never spoke and his face was always concealed with a mask. He was one of the members of the Argent Dawn to help Darion Mograine assault Naxxramas. Darion was however suspicious of Castillian, even some of Darion's comrades in his journey claimed that Castillian was a spy. They didn't have any evidence to support their suspicions so they continued their journey. During the assault, Castillian was believed to be dead.
Weeks passed and it was revealed that Castillian survived the battle and delivers spores of the Forsaken plague to his true master Varimathras who is secretly still loyal to the Burning Legion, confirming that Castillian was indeed a spy of the Legion sent to infiltrate the Argent Dawn. His fate remains unknown after he delivers the spores that finalized the plague.