Specters are hidden guardians to the royal family (Like the sheikian of LOZ). They have varying power but ever one has the ability to shape-shift (They can have preference for some form or some can only be used by certain families). And the demon has slowly been reducing their numbers over generations till it was defeated once more.

Gaster, was the last full blooded Specter. Who took the form of a skeleton to be with his wife. He also was the royal mage/wizard of the king. But had gone missing and is presumed dead. Gaster kept his family's life very private so many don't know he had a wife, let alone children.

The staff he hold is not a soul but a crystal similar to the ones in Eragon that you can use to store extra magic. Staffs can already hold a vast amount of stored magic but the heart shaped crystal increases it's limit to near limitless potential.