The Thorn of Oblivion was a sacred relic believed to hold the souls of all ninjas. Legend has it that if the Thorn were ever to be destroyed, all ninjas everywhere would cease to exist. Oddly enough, it is shaped and colored exactly like a giant candy corn, though it is made of ceramic, and not any type of delicious, sugary substance.
The Thorn was destroyed by The Tick by dropping it a long distance off of a very tall building. While all ninjas everywhere did not suddenly burst into flames, it did bring the grip which the ninjas held clumsily over The City to an end, which is kind of a death, sort of. At least, Arthur thinks so. And he's smart.
The Thorn of Oblivion was one of the driving factors of the plot for the original six issues of The Tick.