"Dangerous" is the second collected volume of Joss Whedon and John Cassaday's Astonishing X-Men, gathering issues #7-12 (originally published 2006-2007) and released as a standalone trade paperback in June 2007.
The story opens with the death of Wing, a young student who loses his mutant powers to the "cure" and takes his own life in the Danger Room, triggering an attack by a damaged, alien-upgraded Sentinel. The X-Men soon uncover a far stranger threat: the Danger Room's training system has achieved sentience, taking humanoid form as "Danger," and Professor X has secretly kept it imprisoned as an unwilling servant for years — a revelation that shakes the team's trust in their mentor. Meanwhile, Emma Frost's hidden ties to the Hellfire Club resurface, adding another fracture to a team already under strain.
Whedon and Cassaday's run reassembled a tight, fan-favorite cast — Cyclops, Beast, Wolverine, Emma Frost and a returning Kitty Pryde — and this arc is widely regarded as darker and more intense than the opening volume, "Gifted," earning the series award recognition and wide critical praise.