Hook: The character group has at least one Sword Bearer in the group. One of these characters' artifacts is more powerful than they can imagine. Currently, the player is still discovering it's power and knows that it allows him, a once normal human, to Mirror Walk.
Line: The artifact has an additional power which it has not made aware to the character for one reason or another. This power would allow the character to dispel the magical barrier between Earth and the Nightlands, allowing the Nightlord minions to cross over in droves.
Sinker: One of the Nightlords (take your pick) has learned of the artifact's existence and wants it for their own dark purposes. By dispelling the barrier between planes, the Nightlords would be able to overtake Earth with far greater ease.
This could easily turn into a full fledged campaign with multiple factions vying for control of the artifact. Some out of belief that the player is too young and inexperienced to wield such power (not knowing that he/she is a Sword Bearer) and that the item cannot be allowed to fall into the wrong hands. Others so that they may have the power of the artifact for themselves.