In "Mean Right Hook" (Marvel's The Defenders, S01E02), Madame Gao and Alexandra have a conversation in which Alexandra mysteriously talks about one thing that the elders of K'un-Lun have valued above all else, from the beginning.
ALEXANDRA: If they wanted to take this one from us forever, they would have destroyed it, not protected it, which means it's not a wall.
MADAME GAO: I've seen it!
ALEXANDRA: It's a door. It's meant to be opened.
MADAME GAO: By what?
ALEXANDRA: The conviction of the elders of K'un-Lun was always unwavering. They think it's a virtue, but in the end, all it ever did was make them predictable. They've locked it away, but a lock is not a lock without a key. And from the beginning, they have valued one thing above all else.
—"Mean Right Hook" (Marvel's The Defenders, S01E02)