Cog first appears as an alley bum, but he seems to know more about Spawn's situation than Spawn himself. Cog is the one who informs Spawn that his powers draw on a limited energy source, and that using it up will condemn him to eternal torment in Hell. He is the angel on Spawn's right shoulder, opposite the Violator, who exhorts Spawn to revel in death and destruction for its own sake.
Eventually, Cogliostro reveals that he is also a Hellspawn, having forsaken Malebolgia long ago, and refuses to use his powers; he has one "tick" of necroplasmic power left, and any use of any of his Hell-granted powers will end his Earthly existence. Still, that single bit of power has extended his lifespan to centuries, and he does not want the newest Spawn to fall to the darkness that spawned both of them.
After an absence in the Spawn comics after #100, Cogliostro returned to Spawn and is revealed that he is dying and that he's terrified of Hell's status ever since Spawn abandoned the throne after defeating Malebolgia. It is then after this that Spawn reveals to Cog of his plan to turn Earth into a new paradise, or Utopia.
Spawn and Cog then travel to the deserts of Tunisia for permission from the Greenworld to transform Earth into Spawn's ideal world until he is attacked by the third Redeemer as an act of vengeance. After the battle, Spawn is abducted by a pair of giant scorpions as they drag him to Hell where he is being awaited by demons.
Still in Tunisia, Cog is visited by Mammon who gives him a mysterious box which Cog seems to recognize and later commits suicide in order to help Spawn down in Hell.
Redeemer, now aware of what he did wrong, travels to Hell to help Spawn battle the demons whose ideas were to battle for the throne of Hell. After a frustrating battle against Violator, Spawn summons all the past Hellspawn before him to help him with the other demons and takes back the throne of Hell.
Cog then runs to Spawn with the mysterious box given to him and tells Spawn that this is what he needs to help him turn Earth into a paradise. Spawn opens the box which turns Hell into an Eden-like world and sees his widowed wife, Wanda, and runs to her, only to be caught in a trap laid by Cog.
It is revealed that Cogliostro was Cain, the first murderer, and since the beginning he never wanted to serve in Hell but to rule it. It was now clear that the only reason Cogliostro helped Spawn was to enable him to gain the throne of Hell, making him the new ruler.
After this act of betrayal, Cain decided to give Spawn "what he always wanted" and sent him to Earth back as Al Simmons without the Hellspawn symbiote attached to him. Cain hasn't been seen since but has been briefly mentioned, such as his plans of building a tower in Hell to pierce the Earth, or perhaps Heaven.