Powdered Rust Monster 3 uses, 0 weight
This thick red slime will rust instantly any metal it comes into contact with. The slime is kept in a brown burlap and silk bag with the lining of a cockatrace’s skin. 1 use is enough to cover a 2 foot square steel or iron object. Roll+Dex, On a 10+ the item rusts away instantly, 7-9; The item rusts but choose one: one of your metal items is affected; You use 2 uses instead of one; the rust slime has gained life and is now a tiny rust creature that could grow. on a 6 or less, the rust get everywhere, choose 2 of the previous.
I like the last option on the 7-9 the best, creating a rust eating ooze for the players to deal with but it may be impractical. Could be powerful but may not be.