The Dark mirror is the tool of a Remembrancer, and is used in Death to See into the past. It consists of two halves that open like a hand mirror, that snap together to create a small metal case. One side has a mirror, for the Remembrancer to look behind them in Death. This is likely signifying that they are 'looking back' in time, in other words, the past. It is also for the practical purpose of allowing the Remembrancer to make sure they are not ambushed by anyone or -thing while they are in Death. The other side is pitch black and does not reflect anything in Life. It is made out of an unknown material. When the remembrancer looks at both sides at once in Death then they can See into the past on the black side. The Dark Mirror is likely a Wallmaker relic.