In Dwarf Mine sign, some of the Darknesses are more dangerous than others. While an outsider with a skeptical bent might see these Darknesses as superstition with the trappings of religion, it seems that Mine sign can in fact conjure quasidemonic entities from the depths of Dwarf mines. The worst of these can take over the mind of a sentient being, turning that being inexorably toward ends of pure vengeance.
When the Summoning Dark invaded the mind of Samuel Vimes, it discovered that vengeance was, for the first time in its remembrance, exorable.
How could that be? What could possibly thwart the will of the most dread and powerful demon of Dwarf lore?
Only the Guarding Dark.
The Guarding Dark springs from the same urges that hold back Vimes - and officers of the Law more generally - from executing foul deeds in the heat of the chase, from doing evil in the pursuit of good outcomes. Crime is done in the dark, while justice requires light; and so the Guarding Dark carries a lantern.
The Guarding Dark could be thought of as an extension of Sam Vimes' Inner Voice, his conscious conscience preventing him from making his own law on the spur of the moment. Or perhaps the Guarding Dark, like the Dwarf Darkenesses, has always existed, and is merely called from the depths when needed.