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.