Complex sentence: In 1998, the Orion Publishing Group purchased Gollancz from Cassell and turned it into its science fiction and fantasy imprint, Gollancz Science Fiction, and in 2005 they set up a manga publishing arm, Gollancz Manga, which publishes UK editions of various Viz Media properties.
Split into simpler sentences: In 1998, the Orion Publishing Group purchased Gollancz from Cassell and turned it into its science fiction and fantasy imprint, Gollancz Science Fiction. In 2005 Gollancz set up a manga publishing arm, Gollancz Manga, which publishes UK editions of various Viz Media properties.