In Assassin's Creed Valhalla, you can find interesting locations where your character will be able to consume not entirely legal substances. In places with mushrooms, you can stop and let the hero take a bite of visions and health problems. This page of our guide describes where the fly agarics can be found in the Rygjafylke region.
You will find fly agarics in the southwestern part of the location.
To complete this side activity you have to kill your opponents. You will fight four of them, each at level 280. If you take on the quest too early and overestimate your capabilities, you can die quickly. Remember, that you can repeat these activities until you succeed.
It is worth to start fighting with full health points and a lot of adrenaline.