This amulet appears to be a +1 arcane focus, and an identify spell reveals it as such.
Any creature that attunes to the amulet must succeed on a DC 15 Charisma saving throw or be trapped, along with anything it is wearing or carrying, in an extradimensional cell. This saving throw is made with advantage if the creature knows the amulet's nature, and constructs succeed on the saving throw automatically.
An extradimensional cell is an infinite expanse filled with thick fog that reduces visibility to 10 feet. The creature trapped in the amulet's cell doesn't age, and it doesn't need to eat, drink, or sleep. A creature trapped within the cell can escape using magic that permits planar travel. Otherwise, the creature is confined to the cell until freed.
If the amulet traps a creature but the extradimensional cell is already occupied, the amulet frees the currently trapped creature to accommodate the new prisoner. The freed creature appears in an unoccupied space near it.
Successful Saving Throw:
- If there is a creature currently trapped inside the amulet, the creature's soul is siphoned for energy to power the "+1 arcane focus" property of this amulet.
- When the trapped creature's soul is completely siphoned (a time determined by the DM), the attuned creature must succeed on a DC 18 Charisma Saving Throw or be trapped, along with anything it is wearing or carrying, in an extradimensional cell. The remains of the previously trapped creature appears in an unoccupied space near the amulet.
- If the attuned creature succeeds it's saving throw the amulet explodes in a blast of negative energy. Each creature within 5 feet of it must make a DC 15 Dexterity saving throw, taking 6d6 necrotic damage on a failed save, or half as much damage on a successful one.
+1 Arcane Focus:
- If the above condition is met, this amulet provides a +1 bonus to the attuned creature's spell attack modifier and spell save DC.
Notes: Bonus: Spell Attacks, Bonus: Spell Save DC, by a sorcerer, warlock, or wizard, Control, Banishment, Utility, Combat, Cursed, Outerwear