Wondrous item, common
This bomb is formed from an explosive device surrounded by the body parts of a rust monster, crushed into a spherical ball. As an action, you can throw this bomb up to 20 feet away. Make a ranged attack against a creature or object, treating the bomb as an improvised weapon. On a hit, the target and each creature within 5 feet of it must succeed on a DC 15 Dexterity saving throw or have its metallic armor begin to rust. A creature wearing metallic armor that fails its saving throw becomes restrained, its armor rusting it in place for 1 minute. The creature cannot take any actions other than to make a DC 15 Strength saving throw to break away the rust, ending the effect on itself on a success.