“‘Cursed,’ Luke said. ‘The water of the lake is in some way poisonous to Shadowhunters. It won’t hurt Downworlders—the Fair Folk call it the Mirror of Dreams, and they drink its water because they claim it gives them true visions. But for a Shadowhunter to drink the water is very dangerous. It causes hallucinations, fever—it can drive a person to madness.’”

When Luke pulls Clary from Lake Lyn, neither yet knows that the lake is the Mortal Glass, so Luke’s description highlights important aspects of Shadowhunter history, as well as differentiating between Shadowhunters and downworlders. Despite keeping careful records of their past, the Shadowhunter community has lost track of the Mortal Glass despite its significance in their history, suggesting that records were deliberately vague or intentionally fogged to prevent Shadowhunters with bad intentions (such as Valentine) from finding and wielding the power of the three Mortal Instruments. It’s unclear why the water is poisonous to Shadowhunters, given that it helped create them, but since it doesn’t harm downworlders, it may have to do with demon energies. Downworlders call the lake by a different name to reflect the effect it has on them. The lake’s prominent presence in Idris suggests that the best hiding place for important objects is often in plain sight.

“‘Not just Valentine,’ said Jace. ‘All of us. The Clave and the Law—what Clary can do overturns everything they know to be true. No human being can create new runes, or draw the sort of runes Clary can. Only angels have that power. And since Clary can do that—well, it seems like a portent. Things are changing. The Laws are changing. The old ways may never be the right ways again.’”

Toward the end of City of Ashes, the second book in Clare’s Mortal Instruments series, Clary draws a new rune that breaks apart Valentine’s ship, something that no other Shadowhunter has previously done. In general, Shadowhunters are a people of traditions, relying on the old ways to inform the future.