This tribalistic brooch looks to be made using a miniature skull, and a quartet of bones tied together to form a diamond-like shape. It looks like it glows when in dim light.

When you cast a spell that deals necrotic damage while wearing this brooch, you deal an additional 1d6 necrotic damage for the spell's damage roll. You can also cast the Bestow Curse spell (DC 14) through this brooch. Once you cast this spell through this brooch, you cannot cast it again until you finish a long rest.

Cursed. This brooch is cursed - attuning to it extends its curse to you. While cursed in this way, your Constitution score is reduced by 2, and any spell you cast that causes a creature to regain hit points instead causes that creature to take necrotic damage equal to the amount it would have healed - when you cast a healing spell in this way, the target may make a Constitution saving throw against your spell save DC, taking half as much necrotic damage on a success.

Notes: Damage: Necrotic, Bonus: Constitution Score, spellcaster, Damage, Combat, Cursed