Throughout the Fnatic mode settings (Settings > Utilities > Gaming mode), the Easter egg is working by tapping on the Fnatic custom logo design five times. Once you do that, a liedertext box will appear wherein you have to the particular text “alwaysfnatic” without the quotes.
Once you do that, the Easter egg is unlocked with a Fnatic logo animation. Therein you will be exhibited three new wallpapers with the Fnatic logo. You can choose to add these meant for default wallpaper collection or just and / or them.
Personally, I’m not particularly liked any of these but if you are and have that OnePlus device that includes the Fnatic mode then you can go ahead try some Easter egg out for yourself.