Hell Butterfly

From Bleach Wiki

Hell Butterfly (地獄蝶, jigokuchō) are completely black butterflies that serve various purposes. They guide the Shinigami through the Senkaimon when travelling between Soul Society and the Human World, thereby saving them from the inconvenience of passing through Dangai. Hell Butterflies are also used as messengers between Shinigami, often carrying important orders. It is implied that Hell Butterflies are bred and taken care of by Shinigami in Seireitei, the latter task being carried out usually by new and inexperienced members of the Gotei 13. Hell Butterflies, like the Shinigami themselves, are only visible to spiritually aware beings.