The Six Death Demon Rings

Introduction
There are only six of these super-rare cursed Rings in the world. The Six Death Demon Rings were created bearing different abilitys which increases the user's fighting strength many times in exchange for a human soul. The Rings power are from a contract with a Demon, The God of Death, or The Devil. The Rings have a will of there own, so they choose the ones they let use there powers.

The Phantom Skull Ring
Owner: Summoner                                                   Ability: Call forth a Genkishi (Phantom Knight) in exchange for a human soul.

The Six Six Six Ring
Owner: Summoner                                                                                                                                            Ability: The user can summon a person to there location as long the user as a blood oath with the person there summoning. The user can also teleport themselfs to any location that they choose as long as they have a six painted in blood at the location there teleporting themselfs to.

The Soul String Ring
Owner: Summoner                                                                                                                                              Ability: Collect the souls of the dead.

History
So........how, what or who made it? How did it come in into your character's possession? Used for good or evil?