Orb of Chaotic Assault

This iridescent glass sphere’s color is constantly shifting. When used as an improvised weapon the orb breaks on impact. If the orb shatters against a creature, one randomly determined resistance or immunity the creature has is reduced for 1d6+1 rounds, after which the creature’s defenses return to normal. A creature’s immunity to a damage type changes to resistance to that damage type, or it loses its resistance. Success on a DC 15 Intelligence (Arcana) check reveals what resistance has been affected after the orb is used.

Source: Level Up Advanced 5e · OGL-1.0a · 5.1