Potion of Diminution is a rare potion in D&D 5e. It does not require attunement.
Potion, rare
When you drink this potion, you gain the "reduce" effect of the enlarge/reduce spell for 1d4 hours (no concentration required). The red in the potion's liquid continuously contracts to a tiny bead and then expands to color the clear liquid around it. Shaking the bottle fails to interrupt this process.