While initially referenced early in the story, a lot of his work is “off screen” which fits his role – working in the shadows.
I wrote the character having left the guild over a disagreement but that was on the surface. Cretus actually left in the story seeing a greater opportunity than honor and fighting the Alliance. He left for the promise and allure of power and advantage from the Legion.
At some point during his departure, Cretus’ travels among the weird realms that warlocks wander in led him to an encounter with another voice.
This voice knew of the Legion’s intentions and had its own plans. Cretus, being an opportunist, discovered and served the whispering mind of N’zoth. He was told, however, to remain in the Legions service and acquire greater mastery over fel.
Returning he delved into the Legions dabbling into the void. Both Socrethar and Xhul’horac had delved into various aspects of each. The Legion had concerns that once they were entrenched in Azeroth they would attract the Old Gods and wanted ways to mitigate their magics.
He shared some of that lesser knowledge with MOK’TRINGAR which led to the creation of the Shadowfel golems. But the greater research from Xhul’horac led him to create a Shadowfel-based curse/plague.
While he worked for the Legion, he furthered his own research and skills. Cretus may have extended his research into other areas of applying shadowfel and to what extent are unknown.
Cretus is presumed dead at the hands of Poisonsword and Prayer but warlocks often have a way of surviving against all odds. Only time will tell whether there is more to Cretus’ story.