Castillian is a mysterious mage of the Argent Dawn. He never spoke, wore a mask to conceal his identity, and was among those who chose to accompany Darion Mograine into Naxxramas to rescue his father, Alexandros Mograine. Darion was somewhat suspicious of him, as was Grunn'Holde — who claimed some thought him to be a Forsaken spy. Though he did nothing to confirm their suspicions in Naxxramas, he did little to allay them either. After being hit in the head by Thane Korth'azz, Castillian was believed dead.
Weeks later it is discovered that Castillian survived his journey into Naxxramas. In a secret meeting in the Tirisfal Glades he delivered a vial of plague based spores to his real master, the Dreadlord Varimathras. These spores became a prime ingredient for the ultimate plague later created by Grand Apothecary Putress.