Disposable procedurally generated dungeon inhabitants, a separate programming class that changelings could never copy — until now.
Procedurally generated NPCs sit at the bottom of the dungeon's classification scheme. The Sluggalos are one such population: throwaway one-offs, barely a step above the NPCs used to populate the clubs, and held in a different classification in the programming from the Class A and Class B NPCs.
That separation is precisely why Justice Light chose the slugs as his sappers — he assumed they were uncopyable by Changelings. Mimicking a throwaway does not sound like a big deal, he notes, but it had never been allowed before, and the fact that it happened at all is alarming on several levels. The breach is the evidence behind his central conviction: if one door is unexpectedly unlocked, other doors are likely unlocked as well.
The disposable NPC tier whose sudden copyability signals that the dungeon's rules are failing.