The Guardians were very mysterious creatures which were appointed to keep watch over the remains of those buried in the sacred Catacombs of the city of Juniper. They did not seem to leave the subterranean chambers, and were never described in any detail beyond the term "monsters" in the Annals. However they must have worked cooperatively on some level with the Custodians, who were the men that were tasked with maintaining the Catacombs and depositing the newly-deceased within.
As there was little or no magic native to Juniper, it does not seem highly likely that the Guardians were magic creatures or demons. They might have been beasts, perhaps even as simple as trained hounds or some other animal. An experienced soldier could kill one even if ambushed by the creature.
When Raven, Marron Shed, and Asa were looting cadavers from the Catacombs during Shadows Linger, a single Guardian attacked Asa and delivered numerous slash or bite wounds. Raven killed the Guardian, presumably with one of his daggers, but continued looting despite Shed's protests. Asa's wounds were bloody but not fatal, and he was nursed back to health later.