Mordenkainen's Faithful Hound

By means of this spell, the wizard summons up a phantom watchdog that only he can see. He may then command it to perform as guardian of a passage, room, door, or similar space or portal. The phantom watchdog immediately commences a loud barking if any creature larger than a cat approaches the place it guards. As the faithful hound is able to detect invisible creatures and ward against the approach of ethereal creatures, it is an excellent guardian. It does not react to illusions that are not at least quasi-real.

If the intruding creature exposes its back to the watchdog, the dog delivers a vicious attack as if it were a 10-Hit Dice monster, striking for 3d6 points of damage. It is able to hit opponents of all types, even those normally subject only to magical weapons of +3 or greater. Creatures without backs are not attacked (ochre jellies, for instance). The faithful hound cannot be attacked, but it can be dispelled. The spell lasts for a maximum of one hour plus 1/2 hour per caster level, but once it is activated by an intruder, it lasts only one round per caster level. If the spellcaster is ever more than 30 yards distant from the area that the watchdog guards, the spell ends.

[Player's Handbook]

School: Conjuration/Summoning
Level: 5
Rarity: Uncommon

Range: 10 yards
Components: V, S, M
Duration: Special
Casting Time: 5
Area of Effect: Special
Saving Throw: None

Material Components: a tiny silver whistle (not consumed), a piece of bone, and a thread (consumed)