Corrode Metal

Choose a metal object that you can see within range, such as a weapon or a suit of heavy or medium armor. You cause the object to become encrusted in a corrosive film which burns to the touch. Any creature in physical contact with the object takes 4d4 acid damage when you cast the spell. Until the spell ends, you can use a bonus action on each of your subsequent turns to cause this damage again.

If a creature is holding or wearing the object and takes the damage from it, the creature must succeed on a Constitution saving throw against the caster's spell DC or drop the object if it can. If it is still in contact with the object, it has disadvantage on attack rolls and ability checks until the start of your next turn.

If the spell has affected a non-magical metal object for 4 rounds of combat, apply a -1 penalty as described the Corroded Metal trait to the object. If the spell affects a non-magical metal object for 8 rounds, apply a -2 penalty. Non-magical metal ammunition targeted by this spell is diminished after 4 rounds of combat as described in the Corroded Metal trait. After 8 rounds, the ammunition is diminished a second time.

At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the damage increases by 1d4 in the initial and any future instances for each level above 2nd.

Corroded Metal. Any non-magical metal weapon that becomes exposed to the Corrode Metal spell begins to deteriorate. The weapon takes a permanent and cumulative -1 penalty to damage rolls. If its penalty drops to -5, the weapon is fully destroyed. A quantity of non-magical metal ammunition that is exposed to the spell is destroyed by an amount equal to the damage dealt by the casting of the spell—for example, if the indicated turn of Corrode Metal causes 10 acid damage, then 10 pieces of metal ammunition are destroyed; this does not take into account the total amount of damage the spell has done on prior or subsequent rounds. If the object exposed is either metal armor or a metal shield that is being worn or carried, it takes a permanent and cumulative -1 penalty to the AC it offers. Armor reduced to an AC of 10 or a shield that drops to a +0 bonus is fully destroyed. To repair such items, one must succeed on a skill check using Smith's Tools against the caster's spell save DC to mend each item individually.

This homebrew spell is among a small selection of Acid magic created for a campaign featuring a player-character who wished to expand their spell list to align with their Sorcerer's Draconic Heritage. Many such spells are attempts to re-flavor existing magic under an acid theme – this spell is a variant of Heat Metal. To view the whole list, please reference my username, Grumbler, while searching. I'm happy to accept and consider any criticism or insight in the development of these spells.