THE KEY. Buffyverse Term/Concept/Idea/Intratext.
Powerful energy that opens the gates between dimensions in a ritual at a specific place and at one moment in time.
The Key appears in Season Five. It is almost as old as the ancient evil god Glory, who intends to use it to return to her dimension. Monks of the Order of Dagon have been its keepers for centuries. Able to transform energy and bend reality, they have started to work on harnessing the Key’s power for the forces of light, but then they realize that Glory is about to arrive to take the Key and kill them. In “No Place Like Home” we learn that, with little time left, the monks had sent it to Buffy in the form of a sister, Dawn, to ensure that Buffy would protect it with her life, and they fabricated memories for all concerned. One monk survives long enough to tell Buffy about this.
In “Blood Ties” Giles’ notes reveal,
All research indicates an energy matrix vibrating at a dimensional frequency beyond normal human perception. Only those outside reality can see the Key’s true nature. The Key is also susceptible to necromanced animal detection, particularly those of canine or serpent construct.
Glory’s serpent-monster (in "Shadow") and mentally ill people (especially those Glory has brain-sucked), who see a bright light, recognize Dawn as the Key. Having been rendered insane by Glory, Tara reveals the Key to be Dawn in Glory’s presence in "Tough Love."
While the monks and then the Scooby Gang seek ways to protect the Key/Dawn from Glory, the Knights of Byzantium are determined to destroy the Key. In “Spiral” their general, Gregor, explains The Key in response to Buffy and Dawn's questions.
DAWN: What about me? What about the Key?
BUFFY: Dawn!
DAWN: I want to know.
GENERAL: The Key is almost as old as the Beast itself. Where it came from, how it was created, the deepest of mysteries. All that is certain is that it's power is absolute. Countless generations of my people have sacrificed their lives in search of it. To destroy it before it's wrath could be unleashed.
DAWN: But the monks found it first.
General: Yes. And hid it with their magicks.
BUFFY: Why didn't they just destroy it? If the Key is as dangerous as...
GENERAL: (interrupts) Because they were *fools*. They thought they could harness it's forces for the powers of light, they failed. And paid with their blood.
DAWN: What do I do? What was I created for?
General: You were created to open the gates that separate dimensions. The Beast will use your power to return home and seize control of the Hell she was banished from.
BUFFY: (laughs) That's it? That's Glory's master plan? To go home?
GENERAL: You misunderstand. Once the Key is activated, it won't just open the gates to the Beast's dimension. It's going to open *all* the gates. The walls separating realities will crumble, dimensions will bleed into each other. Order will be overthrown and the universe will tumble into chaos. All dark. Forever. That is what you were created for.
In “The Gift” Giles says that all hell will be let loose if the Key opens the gates. He explains that the Key as living energy is now Dawn’s blood, and that the gates will not close until it stops flowing, when she is dead.
Once the ritual has started, Buffy decides to sacrifice her own life to save Dawn. She realizes that the monks made Dawn out of her, and that as the two of them share the same blood, her own would finish the Key’s task. On Buffy’s death, the gates close, all returns to normal as there has not been enough time for chaos to take over, and the Key is no longer an active part of Dawn.