I think such thing could exist but i don't like it as an item.
It should be some small objective or task you can do as a part of the map. Currently none of the items interact with the map and I think it should stay that way.
Either an altar or a creep similar to sinners that either costs souls or health to shrink you, either immediately or with a temporary/permanent buff that lets you shrink when you try to enter the tunnels.
It could also be team wide buff, for example a side objective that gave your team access to the tunnels with a movement speed boost for 5 minutes.