Vaughn's run was followed by Robert Kirkman, author of "The Walking Dead" and "Invincible" comic books. Kirkman's run was noted for adapting several major storyarcs from the regular X-Men series. These included Jean Grey's transformation into Phoenix, the arrival of Cable and Bishop, and the appearance of Apocalypse. Kirkman reintroduced Wolverine as an important character, and played with issues of team loyalty. Under his authorship, major characters such as Nightcrawler and Colossus left the team. His tenure also featured Cyclops' decision to turn Xavier's into a more traditional school and consequently disband the X-Men. When this was done an alternate team of X-Men was formed by Bishop as part of the upcoming fight against Apocalypse. After the "Ultimatum" storyline, nearly all of the X-Men were killed, and the team disbanded.