Seal of Phobos

If we assume, the seal was created by Phobos, he must have been the first owner. We definitely know that Alborn was in possession of a Seal of Phobos that he used to open a portal to Earth. Through this portal he helped bring baby Elyon to safety from her tyrannical brother. After that the seal reappears in the cellar under the house that Alborn lived in with Miriadel and Elyon. It was hidden inside the wall within the Book of Phobos. It is possible that they stole the entire book instead of just the seal. After a fight in said cellar, the W.i.t.c.h. girls find both book and seal and the seal ends up being swallowed by the Heart of Kandrakar.

Function: Opening portals to Earth. Apart from that it is possible that the seal was supposed to protect the contents of the Book of Phobos, or that the book is merely the casing for the Seal. Another bigger seal can be found inside the palace that teleports Cedric to another location however it's not sure if the "Seal of Phobos" refers to this teleportation device as well or only to the small item.