Source: Tasha's Cauldron of Everything

Wondrous item, rare (requires attunement by a sorcerer)

This dull, cold crystal sits heavy and leaden, saturated by the Shadowfell's despair. As an action, you can attach the shard to a Tiny object (such as a weapon or a piece of jewelry) or detach it. It falls off if your attunement to it ends. You can use the shard as a spellcasting focus while you hold or wear it.

When you use a Metamagic option on a spell while you are holding or wearing the shard, you can momentarily curse one creature targeted by the spell; choose one ability score, and until the end of your next turn, the creature has disadvantage on ability checks and saving throws that use that ability.