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Callum Sol was a Guardian that wielded the void and member of the Vanguard before getting involved in Shin Malphur's Shadows of Yor and becoming a Dredgen. His reputation paints him as a dangerous individual who delved too far into the Darkness despite secretly working with Malphur to draw out those that would tempt dark powers in order to eliminate the threat they pose.
Callum Sol joined Shin Malphur as one of the founding members of the Shadows of Yor. He and the other Shadows tempted Guardians into seeking darker sources of power to test if they were capable of balancing Light with Dark. Their goal was to find a Guardian tempered to resist the temptation of the Darkness but use some of its power, those who failed the test were killed by Shin Malphur. As Dredgen Cull Callum carried out this task, fostering many new Dredgens along with the other Shadows including the Drifter as Dredgen Hope. Callum became friends with Drifter, before they eventually had a falling out and Drifter made his exit from the Shadows.
Callum had another role to play rather than just Shadow in the latter part of Shin Malphur's plan. He was to be a martyr, pretending to leave the Shadows claiming they weren't going far enough only to be killed by Shin's Golden Gun. His death would serve as a warning and also a final rally for those believing the Darkness was their path. He played his role perfectly, the final confrontation taking place in the Ascendant Realm where the Hero of the Red War is lured along just after to find a planted Ghost shell. The shell has a recording of the confrontation and the Hero of the Red War and Drifter are both led to believe Shin killed Callum without any ulterior motive.
List of appearances