The Key of Lorelei is a powerful weapon created by Lorelei which allows the user to summon the sentience when used alongside the power of the Grand Fonic Hymn. It also allows the user to use Lorelei's power at will. The complete Key is actually two parts: a sword, also called the "Key of Lorelei", which concentrates fonons, and the Jewel of Lorelei, which disperses them.
The Key of Lorelei was given to Yulia Jue as part of her pact with Lorelei. It was used in order to create the Planetary Storm. During the sentience's imprisonment by Van Grants, Lorelei sent both parts of the Key to Luke fon Fabre and Asch. Asch receives the sword itself, while Luke is given the Jewel. However, Luke did not receive anything to his knowledge, and both Asch and the party desperately search for it. At the top of the Tower of Rem, it is revealed that Luke did receive the Jewel, but accidentally absorbed it into his body, as both the Jewel and himself, being a replica, are made of Seventh Fonons. After defeating Asch on Eldrant, Luke wins the sword, the "Incomplete Key of Lorelei". Later, once Luke gains the power of second-order hyperresonance, the ability allows him to complete the Key by fusing it with the Jewel. He then uses the Key and his own power to defeat Van and free Lorelei.