Lake Lyn, also known as the Mortal Glass, is one of the three Mortal Instruments— objects of great power in the Shadowhunter world—of which their mystery and obscurity are emblematic of the elusive nature of power. For centuries, the Mortal Glass has been lost because the Shadowhunters believed it was a traditional mirror. It’s never made clear whether the lake was meant to be the mirror or if the angels disguised the mirror as a lake to keep Shadowhunters like Valentine from abusing the power of the Mortal Instruments—pointing to the book’s engagement with the theme of Power and Its Restrictions. When combined, the Mortal Instruments—mirror, cup, and sword—allow a Shadowhunter to summon the angel Raziel and compel one favor from him. The original Shadowhunter, a human at the time, asked for a way to fight off the demons threatening to overrun Earth. That request birthed the Shadowhunter race and their divine mission to protect the world from evil.

After Clary portals to Idris and lands in the lake, she falls ill because the lake is somehow poisonous to Shadowhunters, perhaps to make the Mortal Glass even harder to discover—another defense the angels set in place to keep Shadowhunters from abusing the Mortal Instruments’ power.