As a player makes their way through The Binding of Isaac: Repentance, they will ultimately encounter a mysterious mirror. Specifically, this mirror makes its appearance in Downpour 2 and Dross 2, and indeed it is possible to pass through it. Detailing how that is done is the precise purpose of this guide, and fans that read on should have no trouble entering the mirror in The Binding of Isaac: Repentance and obtaining the game's first Knife Piece.
The first step to going through the mirror in this major The Binding of Isaac DLC is to locate blue-grey flame in Downpour 2 or Dross 2. This fire is visually quite distinct from the standard blue flames in the area, making it fairly easy to identify. That said, players can get a look at the relevant flame in the video that is featured below, which should alleviate any confusion about exactly what they are to be searching for.
Once a blue-grey flame has been found, The Binding of Isaac players should move directly into it. This will transform them into a character that resembles the Lost, and indeed it is now possible to walk through the mirror. Notably, this character seems to be equipped with something like a Holy Mantle, which will prevent players from dying to a single hit, though it has not be confirmed if this effect recharges upon entering a new room at the time of writing.
Now that the player is on the other side of the mirror, they must simply press forward until they reach the item room that contains Knife Piece 1. Acquisition of that item will in turn give roguelike fans the ability to collect Knife Piece 2, which is accessed from Mines 2 and Ashpit 2 and will serve to complete the Knife. More details on how to obtain that second piece can be found in the next paragraph, and players that want to figure it out for themselves are thus advised to stop reading now.
To put things simply, fans that are trying to find Knife Piece 2 need to activate all of the yellow buttons in Mines 2 or Ashpit 2 and then enter the mine cart in the area. This action will send players through a blocked door, and they can proceed to the item room from there. Fans should take a moment to prepare themselves before they actually pickup the final Knife Piece, though, as coming into contact with the item will cause a weird boss to appear.
The Binding of Isaac is available on PC, Nintendo Switch, PlayStation 4, and Xbox One.