The Thorn is a 2011 Ninjago weapon. It is a simple black spear, said to have been made from the wood of the Sleepy Willow. As such, it can induce sleep in anyone it hits.
Ninjago.com Description
While this spear might not seem like the most dangerous weapon in Ninjago, you would be foolish to underestimate it. Carved from the tired wood of the yawning Sleepy Willow, this weapon will knock you out…. Sweet dreams.