'Messiah Complex' was the biggest storyline to hit the X-Men franchise since the 'House of M'. It's one of those "must read" story arcs if you intend to follow the adventures of any of the X-Men teams from 2008 onward. Messiah Complex is told over 13 chapters, beginning with the X-Men: Messiah Complex one-shot, and then continuing in four of the X-Men monthlys, Uncanny X-Men #492-494, X-Factor #25-27, New X-Men #44-46 and X-Men #205-207. Following on from Messiah Complex, all four 'X-Books' underwent a relaunch, based upon the outcome of Messiah Complex.
To follow the events in Messiah Complex I'd recommend reading 'The House of M' arc as well as the X-Men 'Supernovas' and 'Blinded by the Lights' collections.
The Messiah Complex story really began at the conclusion of the House of M, and the event known as 'M-Day', in which a vast majority of the mutant population was de-powered. The X-Gene itself has all but disappeared, and there have been no new mutant births since M-Day. With mutants bordering on extinction, Messiah Complex begins with the explosive birth, literally, of a mutant. This sparks a race between the X-Men, Mr Sinister and the Marauders, the Acolytes, the Reavers, the Purifiers, and Predator X to see who will get to the child first. Clearly each group has it's own motivations, and the line between good and evil begins to blur in some cases. » Read the rest of the entry..