- "Harry: Do you feel that? It's like...someone's whispering to me...
Hermione: No one's talking, Harry."
- —Harry and his friends, after encountering the Amulet.
The Amulet, initially, was forged from a sapphire gemstone and a golden locket. The Amulet was then transformed into a Horcrux (at that time, known as a "phylactery") when Tyranus murdered the King of Scotland. Over the ages, the Amulet was further enchanted to enhance Tyranus's powers each time he killed or corrupted someone.
In 1899, Tyranus returned to full power after he used Hogwarts' headmaster to power the Amulet which, in turn, granted him "godlike" powers. He battled Albus Dumbledore, a Hogwarts graduate, and was eventually defeated after Dumbledore blew the Amulet from Tyranus's neck and opened its locket, releasing his spirit and leaving him severely weakened.
Years after Tyranus's defeat, Tom Riddle found the Amulet once more and used it as his own Horcrux. It was no longer necessary to murder someone to create the Horcrux as Tyranus had already killed a person ages ago. However, the Amulet was unable to contain both Tyranus's soul and be re-used by Tom as his own Horcrux. This caused for Tom to be "granted" a hideous snake-like appearance but also made Tyranus's spirit slightly stronger.
During the Second Wizarding War, Harry Potter re-found the Amulet in a cave. After he touched it, he was immediately transported to Tyranus's Purgatory "realm". There, Harry made contact with Tyranus's spirit and pleaded him to aid him in defeating Voldemort. Tyranus's spirit disappeared and Harry was transported back into the real world and the Amulet, surprisingly, destroyed itself.