Gateway To The Underworld
The invocation of a very powerful entity named Cerberus is what you are getting with this piece. It is not spirited and does not hold the presence of Cerberus. However, it holds the ability to summon him when you need him and he will show up in his spirit wolf form.
He is the guardian of the Underworld. As such, he acts as the gateway between the earthen realm and the underworld. The Underworld is a place where the presence of many different types of entities are held that are dual in nature-- vampire, incubae, succubae, lycanthropes, warlock, witches, etc. With this piece, you are able to contact Cerberus for his permission to summon these entities. I listed a few, but there are more than what I listed. You will be able to access any entity through the summoning powers that Cerberus will grant to you. In doing this, you will be able to gain the powers and abilities of the entities you summon forth.
In addition, this piece, through Cerberus will allow you to travel to the Underworld in astral from if you choose to. You will take a wolf-dog form that Cerberus will grant to you.