Mordenkainen's faithful hound

Mordenkainen's faithful hound is a conjuration spell that summons a phantom dog for defense and attack. It was presumably developed by the mage Mordenkainen.

Effect
http://www.alchar.org/~aedil/Campaign/WizardSpells/Mordenkainens_Faithful_Hound.html

You conjure up a phantom watchdog that is invisible to everyone but yourself. It then guards the area where it was conjured. The phantom watchdog immediately starts barking loudly if any Small or larger creature approaches within 30 feet of it. (Those already within 30 feet of the hound when it is conjured may move about in the area, but if they leave and return, they activate the barking.) The hound sees invisible and ethereal creatures. It does not react to figments, but it does react to shadow illusions. It is stationary.

If an intruder approaches to within 5 feet of the watchdog, the dog stops barking and delivers a vicious bite (+10 attack bonus, 2d6+3 points of damage) once per round. The dog also gets the bonuses appropriate to an invisible creature. (For most defenders, the invisible creature gets a +2 attack bonus and the defender loses any Dexterity bonus to AC.) The dog is considered ready to bite intruders, so it delivers its first bite on the intruder's turn. Its bite is the equivalent of a +3 weapon for purposes of damage reduction. The hound cannot be attacked, but it can be dispelled.

The spell lasts for 1 hour per caster level, but once the hound begins barking, it lasts only 1 round per caster level. If you are ever more than 100 feet distant from the watchdog, the spell ends.

Material Components
A tiny silver whistle, a piece of bone, and a thread.