From DownSights: “A lot of unique content was added to Call of Duty: Warzone with Season 6, including a brand new easter egg that utilizes the new subway system. Completing the easter egg rewards players with a beautiful Bruen MK9 blueprint called Firebrand. In this guide we will cover everything you need to know about the easter egg, so that you may also obtain the new Firebrand.”
Source: N4G PC Secret Subway Easter Egg In Warzone – New Bruen 'Firebrand' Weapon Blueprint