Vassago, in demonology, is a mighty Prince of Hell (see Hierarchy of demons), ruling over twenty-six legions of demons. He can be persuaded to tell the magician of events past and future, can discover hidden and lost things, and has a "good" nature.
Vassago plays a significant role in the novel The Day After Judgement by James Blish, where an ambiguous relationship to the majority of the Infernal forces is implied. A psychotic serial killer character takes on the name Vassago in the novel Hideaway by Dean Koontz.
The Third Spirit is a Mighty Prince, being of the same nature as Agares. He is called Vassago. This Spirit is of a Good Nature, and his office is to declare things Past and to Come, and to discover all things Hid or Lost. And he governeth 26 Legions of Spirits, and this is his Seal.Joseph H. Peterson - The Lesser Key of Solomon
Vassago has sometimes been called a Prince of Prophecy.
There are some ancient descriptions of the demon and its appearance. Below are a few quotes from various magicians who have called on this demon spirit. Fact or fiction, they are interesting...
One early description of Vassago was that he appeared "as an aged man riding a crocodile and carrying on his right wrist a Goshawk. His eyes are hollow and see into other dimensions, but he is blind in this dimension."James Donahue - Vassago: An Impish Spirit
Still another description is that Vassago appears as "a blood-red dragon, extending one slithe (30 feet) long." The creature was observed by this summoner as having large, red wings. It walked on four legs, had green eyes and white fangs.James Donahue - Vassago: An Impish Spirit
Vassago enjoys changing form and has been know to play games with magicians that call on him. Once seen as a dark and menacing entity, in recent years he is now being thought of as being, playful, and mostly harmless.
There are dozens of books on the subject of magick and the Keys of Solomon. This is a short list of some of the more informative books relating to the subject.