"This creature has the body of a large dog, with a smooth black coat and a mouth full of sharp teeth."
Shadow mastiffs are outsiders from the Plane of Shadow. They prefer fighting in shadows or dark conditions, which gives them a great advantage. If a magical light source negates the shadows around them, shadow mastiffs are cunning enough to either move out of the light or back off and break up the opposition with their baying. They have been known to seize and carry off items enspelled with daylight spells.