The Nightosphere amulet first appears in the episode "It Came from the Nightosphere" as part of Hunson Abadeer's clothing. In "Daddy's Little Monster," it is revealed that it gives Hunson his powers and turns anyone who wears it into a powerful demon. Hunson Abadeer falsely tells Marceline that "it'll grant you wishes" to get her to take over the Nightosphere.
The amulet is a purple rhombus-shaped gem hanging by a gold chain.
The amulet transforms the wearer into an evil demon in a black business suit. The amulet's effect on the wearer's appearance varies: when Hunson Abadeer puts it on, he becomes thinner; when Marceline wears it, her head becomes larger, heart-shaped, and purple, with small darker purple tentacles growing from her lower face, red horns, and a vertical-opening mouth—a head very similar to her father's after he transforms in "It Came from the Nightosphere"; when Finn wears it, he grows to a tremendous size, and his head becomes large, round, and light purple. He also grows dark-purple and light-blue tentacles. His bulging eyes appear to have no openings and have purple tentacles coming from the bottom of them, and he grows four tentacles.
Wearers of the amulet also gain the ability to suck the souls of the living. The wearer will lose all powers and their demonic form once the amulet is removed. The wearer can also turn a demon into a group of smaller demons the same way Flame Princess can turn fire elementals into Flambits when in cloud form, another power that comes with the amulet. Whether that power is exclusive to Marceline is unknown.
- The amulet has been worn by Hunson Abadeer, Finn, and Marceline. Although Finn says he is "pure of heart," the amulet turns even him evil (and gives him trauma).
- The most notable change the amulet has on its wearer is changing their alignment to Chaotic Evil. Hunson Abadeer seems completely unaffected by this (presumably because he already is Chaotic Evil; however, he does act nicer without it on in "Return to the Nightosphere"). Marceline becomes completely unstable, enacting punishments on the citizens of the Nightosphere at random. Finn briefly retains his heroic resolve, helping Marceline and Jake escape, but moments later is completely taken over by the evil and tries to return Marceline to the Nightosphere.
- The amulet may also harbor restoration magic, as it can easily be pulled away from the person's neck from the back and not be damaged or cut.
- The amulet resembles the Holo-pendant that Princess Bubblegum gives Finn in "The Other Tarts."
- When Finn and Hunson Abadeer take off the amulet, the evil and chaotic matter is absorbed back into the amulet. When Finn and Jake take it off of Marceline, the evil substance falls off and soaks the demons, who eat it.
- Notably, the only times the chaotic substance goes back into the amulet is when Hunson takes the amulet off himself and Finn. This is another difference with Hunson Abadeer for the amulet, though this may be a coincidence.
- Hunson is the only one whose voice is not altered in any way by the amulet.
- Finn's form while wearing The Nightosphere amulet is a parody of a famous horror game Slenderman.