The Serpens Clavem (also called "the Key"or "portal key") is a mystical object used to open portals to different dimensions, and especially one between Asgard and Earth. But unfortunately, it was an object full of dark magic, capable of corrupting even the best people.
The key takes the form a golden snake with very detailed features, from the scales to the eyes made with two shining ruby stones. Its tail ressembled the end of a key, to display its opening abilities.
The Serpens Clavem was forged thousand years ago in Asgard, and nothing is known about its use until it came into the hands of King Nikolaus, whom it started to possess. Completely corrupted by the snake and maybe fueled by its power, Nikolaus took the throne in Asgard and won a war that had started after his accession to the throne. When those who opposed the King - among which were Joanna, Victor, Wendy and probably Ingrid, Freya, Archibald, Vidar and Harrison - lost the war, they fled and took the key to cross over to Earth and sealed the portal off behind them.
And at some point, in 1906, Archibald Browning and his cult came in possession of the key and wanted to use it so that they could go back to their dimension and leave Earth for once and for all. Ingrid and Archibald both died before they could finish their plan and open the Portal. After that, Archibald's daughter Athena was sent away and Joanna and Wendy took the key. Even though Joanna promised she would destroy it, she in fact decided to keep it, as it was her last link to her son Frederick, who had stayed behind in Asgard.
A century later, Athena, feeling that the key had not been destroyed, came back into East Haven to seek revenge on the Beauchamps and to finally fulfill her father's dream. To get the key, she raised the 1906 Ingrid back from the dead and set her against Wendy to discover the whereabouts of the key - but Joanna came in time before Wendy could say a thing (even though she was not even aware of it). The day after, Joanna admitted the truth to her sister: that she had not destroyed the key and had kept it in her garden in case she would ever use it to retreive Frederick. But Wendy got possessed by the power of the key and a battle ensued between the sisters, which forced Joanna to choose between her sister and the key. She chose Wendy and therefore finally destroyed the key.
The key is first of all an opening device, capable of opening doors between dimensions, and especially between Asgard and Earth. But, it was also a vessel of negative energy, capable of possessing people. When looking into its ruby eyes, the snake would start talking to you, until you gave in its lies and wanted to use it at last - from this moment, it would fix itself on the person with its fangs and start injecting venom in their blood, slowly corrupting and burning away the good in them.
While possessed, a person's eyes would turn dark and the person would also display enhanced physical abilities and magical abilities.