Thread is dormant in the extreme cold of outer space, but reactivated by passage through the atmosphere. When it lands on a solid surface it burrows into it, multiplying extremely rapidly and killing any living organisms that it touches. A burn caused by thread is called "threadscore".
Although deadly, thread is not indestructible. The primary means of defense against the thread are the dragonriders and their mounts, who flame the thread in midair before it can land. Thread drowns if it lands in a body of water, and it can be destroyed by spraying them with "agenothree" (nitric acid - HNO3) before they burrow into the soil.