Oil of Diminution is a Consumable(Coating). It can be applied to a weapon to give it special properties for ten turns or thrown as a Grenade to inflict its effects in an area.
Gazing through the glass bottle, everything seems shrunken and contracted - almost gaunt.
Properties
Single Use
Rarity: Uncommon
Weight: 0.1 kg / 0.2 lb
Price: 30 gp 
UID
ALCH_Solution_Oil_Reduce
Effect
Bonus action
Coat your active weapon in Oil of Diminution, giving it a -1 Weapon Enchantment.
Condition: Coated in Oil of Diminution
Duration: 10 turns
- Targets must succeed a DC 11
Constitution saving throw or receive 
Disadvantage on 
Strength checks and Strength saving throws, as well as a -1d4 penalty to damage on melee attacks for 2 turns.