Obtained from an Anthropterist, a Collector’s Rod can be used to shrink and grow a target as an action. The action involves fixing the target in the center of the lens and utter a control phrase, such as “I’d like you better half your size” or “a little bigger, if you please.” Thus, it requires line of sight and cannot be used when silenced. It has a range of 20′. If it suspects anything, a living target may make a DC 16 Dexterity save to escape the scope of the lens before the phrase is completed to avoid its effect.
The minimum size to which a target can be shrunk is 1/25th, the maximum to which it can be grown is double.
When a living target’s size is changed, it makes a Constitution save with DC of 10 +1 for each 10% change. On a failed save, the target falls unconscious.
When used by a non-fey, the user becomes burdened with invisible, intangible weight equal to the change in the target’s weight. This weight only increases again when the change is reversed.
Upon use, it attunes to the user and cannot be gotten rid of (along with the weight) except by giving it to a fey who willingly receives it, likely also expecting some other gift or favor in return to taking the burden. Once the rod is unattuned, the burden is removed and all the things changed by the user with it return to their normal size.
If you liked this item, you can find the methodology used to develop it in my document on items that are puzzling to use: A Blessing and a Curse.