The Mortal Mirror, also known as the Mortal Glass, is in fact Lake Lyn in Idris. It is the third of the Mortal Instruments given by the Angel Raziel to Jonathan Shadowhunter, the first of the Nephilim.
Ingesting the lake's waters can be poisonous to Nephilim, but it has no effect on Downworlders. Faeries have been known to drink from the lake, saying that it gives them true vision; for the Nephilim, the water causes hallucinations and may even drive them to madness.
Prior to Jonathan Shadowhunter's encounter with Raziel at the lake, it was said to not be the glittering blue body of water of today, but instead a black roiling tear in the fabric of the world, through which demons passed back and forth freely.
For a long time, the whereabouts and information regarding the mirror remained a mystery for the Nephilim, although the Lake Lyn has always been a part of the original legend of the Mortal Instruments. Lake Lyn was said to be the lake from which Raziel rose, holding both his cup and his sword to be given to the new race of warriors he created. Since Raziel's summoning at the lake, it has been said to have been cursed.
In 2012, the water from Lake Lyn was discovered to be able to stop the spread of the blight and its transformative effect on the warlocks. Large amounts of the water were retrieved and distributed by Diana Wrayburn and the Wild Hunt.
- Among the Fair Folk, the lake is known as the Lake of Dreams or Mirror of Dreams.