The thread circle was a powerful magical artifact that was made of four types of ambient magic: thread magic, green magic, metal magic, and weather magic. In its first form, it was a string of thread woven into a circle with four lumps equidistant from each other. In 1043 KF it ceased to exist physically, and instead became an intrinsic part of each of the four mages who imbued it with their magic.
The thread was first created in 1035 KF by Sandrilene fa Toren under Lark's direction and was the first thread she ever spun. The four lumps that had been part of the thread were because she was thinking of new life while she was spinning.
Sandry spun the thread together later in 1035 when an earthquake devastated the city. They were trapped in the ground and needed strength to keep from getting crushed by the rocks above them. Each of the four imbued a lump with their own power. This is when Sandry first exhibited her ability to spin magic. This ability might have been enhanced by the thread circle, as later she was able to spin pure unmagic and didn't have to use a physical thread.
The creation of the thread circle successfully increased their power exponentially. After weaving this, they were able to mindspeak, to see magic, and to sometimes share different aspects of their magics. They were also able to break into pattern magic—a feat that is impossible according to Niklaren Goldeye; to weave fire; to create a living metal that could grow and change; and to rescue someone from death.
Their individual magic also heightened in power. Sandry fa Toren can weave magic—an ability that she is believed to have possessed even before the thread circle was created. Briar Moss could destroy an entire mansion without breaking a sweat; Daja Kisubo continued making living metal objects, including gloves that could withstand fire and mold to someone's body; and Trisana Chandler could see things on the winds as well as use her magic for incredible feats.
In 1043 KF, the four brought down an impenetrable magical barrier on the Namorn/Olart border. It was believed to be absolutely infallible, and they used their thread circle to destroy it. Tris was able to join them remotely using their shared magic, having been injured due to a curse set by Ishabal Ladyhammer. Using the thread, they were able to draw immense power from natural sources in order to destroy the magical wall. Briar drew power from roots of the forest near them, algae from Lake Glaise, the forests on the Carakathy Mountains, and the grass plains where they stood. Daja took her power from the layer of ore deep in the earth, and from magma and molten metal. Tris was able to garner power from the air above the mountains and drew from the movement of lava and the pressure of water from cracks in the earth. As the power of the magical barrier flew at them, Sandry wove magical nets to capture it.