"Witch Queen" is a side quest available in Dying Light.
Dahlia will give you a task of going to a nearby cave which can be accessed through underwater pathway in nearby canal. At the entrance is an drop crate. You need to collect 10 mushrooms growing all over the caverns. Make sure to go there during the night, because during the day there are a bunch of volatiles who will surely kill you.
Once you do that go back to Dahlia and collect the potion and blueprint. The potion has a side effect of reversing backward and frontal movement.