Here's a synopsis that outlines the core premise of your story:

A powerful demon, in a moment of arrogance or oversight, makes a critical mistake while handling a potent, ancient gem – a source of immense power, including immortality. This error results in the gem being lost and inexplicably transported to the human world.

Unbeknownst to her, the gem finds its way to the neck of a talented and rising fashion designer. As the gem's power subtly integrates with her, she begins to experience unusual abilities, most notably an uncanny resilience and perhaps other subtle enhancements.

Meanwhile, the demon, desperate to reclaim his lost power and avoid the wrath of his superiors, is forced to venture into the human realm, a place he finds both fascinating and frustrating. He discovers the gem's location and realizes the complicated situation: it's bonded to this human woman.

The gem's presence doesn't go unnoticed by others. Word of its power, whether through whispers of the demon's arrival, strange occurrences around the designer, or ancient prophecies, begins to spread. A diverse group of individuals, each with their own motives – from power-hungry sorcerers and shadowy organizations to desperate individuals seeking eternal life or simply immense wealth – begin to converge on the fashion designer, making her an unwitting target in a dangerous supernatural game.

The fashion designer, initially unaware of the true nature of the gem she wears, finds herself thrust into a world of demons, magic, and perilous pursuits. She must navigate this dangerous new reality, understand the powers she now possesses, and protect herself from those who would take the gem, and potentially her life, to claim its power. The demon, torn between retrieving the gem and perhaps developing an unexpected connection to the woman who wears it, adds another layer of complexity to the perilous situation.