Item from Undertale / Deltarune by Toby Fox
Mysterious dark crystals dropped by secret bosses — collected by Seam, who warns that gathering them all will reveal something terrible.
Shadow Crystals are Deltarune's meta-collectible — artifacts dropped by secret bosses that Seam collects in his shop. Each secret boss was a character who learned something about the true nature of the world and was broken by it. The crystals they leave behind may contain that knowledge. Seam accepts them with weary resignation, warning that completing the collection will reveal something terrible. Whether 'terrible' means dangerous or merely devastating depends on what the crystals ultimately reveal about Deltarune's reality.
Small, dark crystals that pulse with an inner light. Each one is dropped by a defeated secret boss — Jevil, Spamton, and presumably others in future chapters. They fit together like pieces of a puzzle nobody wants completed.