You've retrieved the artifact, as the demon requested. It's abundantly clear that the artifact has been corrupted. If you bring this to the demon, he will most certainly use it for a great and terrible evil. But you're prepared to betray the demon and fight this evil, whatever it may be.
Why take the risk at all? You sealed the rift without handing over the artifact to the Demon. Clearly it was not to be trusted with such a dangerous and powerful item.