Demon dogs seem to follow a pretty consistent power set, which is fine because it lets authors play with the archetype rather than explain everything from scratch. They're almost always tied to shadow or death magic, able to blend into darkness and strike from nowhere. I've noticed a strong link to necromancy or soul-eating too; they don't just kill you, they consume your essence to empower their master. Physical abilities are a given—enhanced speed, strength, claws that can tear through armor. But what's more interesting is the pack mentality they often bring. A lone hellhound is scary, but a story where the demon dog is the alpha of a spectral pack adds a whole layer of strategic threat. They become a mobile, intelligent unit rather than just a monster.
Some novels get creative with the 'demon' part, moving beyond fire and brimstone. I read one where the dog's bark could impose curses, like a temporary paralysis or a luck-drain. Another had its fur act as a focus for teleportation magic, letting its summoner instantaneously travel to its location. The loyalty aspect is a power in itself, really. That unshakable bond with a protagonist, often a dark mage or a revenge-driven antihero, turns them into a narrative device for foreshadowing danger or showing a softer side. The dog senses ambushes the human can't, or refuses to leave a wounded master's side, which tells you more about the character than any monologue could.
My personal favorite twist was in a web novel where the 'demon dog' was actually a corrupted celestial hound. Its powers were inverted: it could purify low-level undead by touch but was lethally vulnerable to holy water. That subversion of expectations made it memorable. Usually, though, they're the guardians of thresholds—hellgates, tomb entrances, that sort of thing. Their presence signals you're crossing into a realm where normal rules don't apply, which is a classic and effective bit of atmospheric storytelling.