You create a phantom watchdog made of magical force, which is invisible to everyone but yourself. The hound guards the area you designate, patrolling the entire length and breadth of it, though it never moves past the bounds of the spell’s effect.
If any Tiny or larger creature (or any swarm, whatever the size of its constituent creatures) enters the area, the hound automatically perceives it and begins barking loudly. (Creatures within the spell’s area at the time of its casting never trigger its barking or attacks, but intruders who leave and then re-enter are treated just like new intruders.) At the same time, the hound moves to a space adjacent to the intruder (it can disappear and instantly appear anywhere within the spell’s area) and attacks. The hound is considered to have a readied action to attack an intruder; in subsequent rounds, it attacks once per round.
The dog’s bite is considered to be a +2 weapon for the purpose of bypassing damage reduction. Its attack bonus is equal to your caster level plus your spellcasting ability modifier; it also gets the bonuses appropriate to an invisible creature. Its bite attack does 2d6 points, plus your caster level, of bludgeoning, piercing, and slashing damage.
As a force effect, the hound can strike ethereal and incorporeal creatures, doing full damage to them. It sees invisible and ethereal creatures. The dog does not react to figments, but it does react to shadow illusions.
Creatures bitten by the dog find themselves dimensionally anchored? (spell resistance applies against this effect; check only once for each intruder). If an intruder attacked by the hound moves past the bounds of the spell, the hound cannot pursue, and ceases its attacks. While the hound is attacking an intruder, it continues barking loudly.
The spell lasts for 1 hour per level, but once the hound begins barking, it lasts only 1 round per level. If you are ever more than 100 feet from the bounds of the faithful hound’s area, the spell ends immediately.
The hound cannot be attacked, nor can it be dispelled, but any effect that brings down a wall of force? also destroys the dog (its AC against touch attacks is 10).
Two or more faithful hounds cannot share any part of their area; if you attempt to conjure a faithful hound such that its area overlaps that of an existing one, the new spell fails.
Material Component: A tiny silver whistle, a piece of bone, and a thread.