Non-Canon Alert! The subject of this article originates from non-canonical sources. To find out about what is considered "canon" see LOTR:Canon.
Sebastian was a hedgehog appearing only in the Peter Jackson movie The Hobbit: An Unexpected Journey (2012). He was living near Rhosgobel at the edge of Mirkwood forest. Radagast seemed to have a close bond with this creature. He found Sebastian on the forest floor apparently dying. Radagast soon found trees and plants and animals dying all through the forest. When Radagast saw Sebastian squirming on the ground, he knelt down and grabbed him, brought him back to his house, and there tried in vain to heal him using various potions and incantations. When Giant Spiders crawled over his house, Radagast realized dark magic was at work. Radagast was able to revive the hedgehog using the stone from his staff.
Neither Sebastian nor any of the incidents involving Radagast in the movie were ever mentioned in any of Tolkien's writings; they are a complete Peter Jackson invention for the movie. In Tolkien's writings the forest had become darker, thicker, and infested with spiders, bats, and orcs after the Necromancer (Sauron) occupied Dol Guldur in the southern part of the forest, but there was no mention of any magic that started poisoning all the animals.