Even though the ancient city of K'un-Lun has been destroyed, its secrets keep unfolding. Spoilers for The Defenders Episodes 1-5 ahead. The mystical realm where the Iron Fist trained may have much more to give. What's the "substance" in The Defenders? The Hand needs something from Danny Rand, and Alexandra may have casually dropped some important information.
In Episode 5, Sigourney Weaver's character tells Elektra a story from her past. She had a daughter once, she says, "before I made the pilgrimage to K'un-Lun. Before I learned of the substance, and before I started the organization." The story was about how she learned to conquer death after having already lost her daughter. It seems safe to presume that the substance is related to immortality.
Now, compare that to Stick's tale about the origin of The Hand. He told Danny, Luke, and Matt that the five heretics in K'un-Lun were uninterested in using chi to heal and put good in the world. They wanted power. They wanted to defeat death.
The Hand has also made it pretty clear that they want Danny Rand alive, because they need something from him — something related to his powers. Is the Iron Fist chi the substance? Can such a thing be extracted from a human being? Is it even tangible?
K'un-Lun isn't known for any natural resources in the comics. The only significant substances in Marvel comics are metals like vibranium and adamantium. Maybe the substance is unrelated to Danny, and just something found in the city of K'un-Lun like a magical mud mask or a freshwater spring. The substance could also be the Black Sky itself. The Hand is fond of referring to Elektra as an inanimate object.
Whatever it is, Alexandra's mention should not be taken lightly. Danny Rand was trained to fight The Hand, but he may not have expected them to pursue him and his abilities just as fervently. This substance, whatever it is, probably has something to do with their pursuit.