No one knows who the Weapon XI character is, and this move is very intriguing. Wikipedia offers us some insight:
No individual with the ‘Weapon XI’ moniker has been seen, but John Sublime has mentioned it. A likely candidate for Weapon XI’s true identity is the Hound (a mutant with the power to hunt and kill other mutants), who was transformed by the Hound Program of the US government into a techno-organic killing machine. He battled the government-sponsored incarnation of X-Factor after massacring scientists at a US government research facility, nearly killing Sabretooth in the process, but was stopped by Polaris, who was forced to use her power at its full limit. He was later after Shard, but was destroyed in an explosion after being defeated by her and Bishop.