This artifact is a monstrous head mounted on a wooden plaque.  The plaque if examined is nearly a thousand years old, and inscribed on it in Latin are the words "The head of the Beast" and "From its mouth, wisdom".  This is the head of the demon Baphamut who legends say the Templars asked questions to and it responded.

Historically, the head is almost certainly a myth.  Rumors of devil worship were actually common in the middle ages and saying that someone practiced the kind of black magic that would have them keep a monsterous head on their wall that answered their questions was pretty believable to people in those days.  However, much like any wild rumor with no basis in fact, there is no solid information about the head.  Descriptions vary widely, from it being a wooden head to being made out of silver to it being a human head to it being a goat's head.  The oracle will proably have to be flesh, if only to allow for movement, but you might decide that something more exotic fits your campaign.  Also, the head could be of a particular Mythos creature if you wanted to tie it a bit more closely to the Mythos.

When someone who knows how to use the head asks a question to it, they spend a number of magic points equal to the number of words in the response.  The head will be terse, giving one word answers if possible.  SAN cost for someone doing this will can depend on the situation, but 2 SAN for activation should be enough for most.  Increase it for difficult or more complex questions just to discourage players from using it.  Also, seeing the head move and answer costs 1d4/1d8 SAN points (include the person asking the question in this).  Finally, the head drains a point of POW from the user which can be recovered with rest, and naturally anyone dropping to 0 in such a manner will die.

If someone does not know the purpose of the head, activation does not have to be automatic.  You have to willingly try to get the head to respond.  If someone just casually asks the head a question, give them a luck roll to see if it responds.  Since they would not want the head to activate normally, failing the luck roll would activate it (of course, if they're getting vital information in this method, making them succeed would be more appropriate).