Twinleaf’s Oxidising Touch

This spell has no effect on magical items.

You attempt to parry an attack from a metallic weapon with your bare hands. Make a dexterity saving throw against a DC of X+7, where X is equal to the amount that the attack exceeded your AC by.

On a success you catch the weapon, and all metallic elements of it you touch rust instantly and are destroyed. Any damage associated with the attack from that weapon is negated. 
On a failure your hand glances against the weapon and rust spreads through its metallic elements. The following effects are applied based on the damage type the weapon inflicts:

• Slashing; you corrupt the edge of the blade. Damage from the attack is halved, and the damage from all subsequent attacks with that weapon are also halved.
• Bludgeoning; you poison the solid core of the weapon. None of the damage of the attack is mitigated, but on its next hit, the weapon shatters and is destroyed. The damage of that attack is halved.
• Piercing; you dull the sharpened tip of the weapon. Damage from the attack is halved, and all subsequent attacks with that weapon roll with disadvantage.

This spell can also be cast using an action with the following effect:
You touch a non-weapon metallic object no larger than a one foot cube. Rust spreads through the metal, embrittling it and lowering the DC to destroy it with a Strength check by 5. This has no effect on shields or metallic armour.

The effects of this spell are permanent,  but can repaired by a blacksmith or other appropriately skilled craftsman.