Magnetite Shard

3rd-level evocation

Casting Time: 1 action
Range: 90 feet (20-foot-radius sphere)
Components: V, S, M (a piece of magnetite)
Duration: Instantaneous
Class: Druid, Sorcerer, Warlock, Wizard

You fling a supercooled piece of magnetite at a point you can see within range, where it detonates in an explosion of icy, metallic shards. Each creature in a 20-foot-radius sphere centred on that point must make a Dexterity saving throw, taking 4d6 cold damage and 3d6 piercing damage on a failure, or half as much damage on a success. Creatures made of a ferrous material or wearing ferrous armour have disadvantage on this saving throw.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the cold damage increases by 1d6 for each slot level above 3rd.

Source: Heliana's Guide to Monster Hunting