In Dead Cells, you later obtain the Hunter’s Mirror from the Specialist’s Showroom. Although the game doesn’t explicitly explain its purpose or use, the mirror is a very useful tool—as long as you know what it’s showing you.
The Purpose of the Hunter’s Mirror
The Hunter’s Mirror will show up at the Prisoners’ Quarters at the start of every run after you unlock it. You can do this by purchasing its blueprint at the Specialist’s Showroom, then using that to unlock it from the Collector for 150 cells.
Upon first use, the Hunter’s Mirror is completely vague. It will show you an enemy without explaining why. However, if you manage to find that enemy type and defeat it, it will drop a blueprint you haven’t discovered yet.
That is the true purpose of the Hunter’s Mirror. You can use it to find more enemies that hold blueprints you’ve yet to acquire.
Each time you find a new blueprint from an enemy that the mirror has shown you, return to the Hunter’s Mirror to find more.
Whenever you start a run to seek out a blueprint, be sure to carry a Hunter’s Grenade with you. Once you find an enemy that was shown by the mirror, throw the Hunter’s Grenade at it to turn it into an elite variation of itself. This allows you to have a higher chance of obtaining rarer blueprints from them.