Key Thread (鍵の糸 Kagi no Ito ) was an Unrestricted Spell planted in Torches in Osterode by the leader of Töten Glocke, "Weaver of Coffins" Asiz, several hundred years ago in the Middle Ages in preparation for the activation of City Devourer.
The Key Thread was first planted inside Torches created by the Unrestricted Spell Master. When the master activates Key Thread, the Torches lose their function as replacements and disintegrate, returning to Power of Existence.
Chance Meeting ArcEdit
In modern times, "Hunter" Friagne planned to recreate City Devourer in Misaki City using a process similar to Key Thread, namely simultaneously exploding many Torches with the handbell Treasure Tool Dance Party.