- "When the key that overturns all is possessed everything shall unfold."
- — Harald Hoerwick —
The Key of the Twilight (黄昏の鍵, キー・オブ・ザ・トワイライト; Tasogare no Kagi, Kī obu za Towairaito), sometimes called the Twilight Key, is a legendary item rumored to have the ability to bypass the system in The World.
It was first mentioned in .hack//SIGN and was sought out by the main characters of the series. By the time of SIGN, the rumors and quest for the Key had long subsided, but the appearance of Tsukasa and the monster he commanded renewed interest in it as theories linking him to the Key were spread quickly. Analysis refers to the events of SIGN as the "Key of the Twilight Incident".
The search for the Key is an important part of the series, and there are many discussions as to whether Tsukasa possessed or actually is the Key. Mimiru feels that the phenomena surrounding Tsukasa are entirely detrimental and have no conceivable purpose, but Bear counters this with the argument that apoptosis, a biological function that kills cells, is actually necessary for growth and that the Key's function as something that goes against the system may actually be used to benefit the system. By the end of the SIGN series it is suggested by Bear that Aura may be the Key of Twilight. This viewpoint is shared by Sora who specifically calls Aura the "Key of the Twilight" upon seeing her for the first time.
In the end, it is found out that Aura is the Key of the Twilight.
The definition of the Key of the Twilight is ambiguous. In .hack//Roots during Ovan's discussion with Naobi, it is revealed that the Key of the Twilight has taken the form of a Twilight Bracelet and the Ultimate AI.
Ovan defines the Key of the Twilight as "a guide to truth". Any item with the power to lead to the truth behind The World is thus dubbed "The Key of the Twilight". The Twilight Bracelet and eventually Haseo's PC are both examples of such a power, and were balanced by the anti-existence Cubia.