Ghoul Slugs were Slugs that had been corrupted by either using Dark Water or a Goon Doc. Ghoul Slugs can be turned
back into regular Slugs by healer Slugs or The Lightwell. Dr. Blakk uses a small machine that gases a Slug and then transforms it. Some SlugSlingers prefer Ghouls over Slugs because they are more powerful than standard Slugs, even though Ghouls are hard to control and prone to running away once fired from a blaster. While Ghouls are stronger than regular Slugs, it has been seen that a highly trained Slug can defeat them. This indicates that Ghouls are not automatically stronger than Slugs, as seen in "The Trade", where Blakk offers to give Eli his Slug back in a stronger state. Overall, the strength of a Ghoul is dependent on the overall strength of the Slug's original power. A Ghoul can probably get stronger through training like normal Slugs, though this is unconfirmed. When Ghoul are shot out of a blaster, they tend to run off and form wild packs but this does not seem to line up with the show where C.C. is seen calling his scattered Ghouls back by a whistling. It is possible that the Ghouling process makes Slugs less likely to listen to their Slinger, requiring a stronger bond in order to control them.
It has been seen that the creation of Ghouls can cause devastating effects on Slugs. One such case would be Burpy in the episode "Lightwell", whose power was quickly being drained by the dark energy of Ghouls making him very sick and unable to transform when shot out of a blaster, forcing him to visit the Lightwell.
As seen in "Back to Blakk" a large number of Ghouls can open a portal for the Darkbane, but only four can pass through and the first ghoul was actually a Greneater.
List of Ghoul Slugs
There are 26 different kinds of Ghoul Slugs so far in the show: