After the death of Lawrence Trask, the property of plans for the Sentinels fell to the government of the United States. Dr. Stephen Lang became the head of a federal research about mutants, trying to discover how mutations could provide superhuman powers. However, unbeknownst to his employers, Lang was an anti-mutant bigot who used his position to obtain access to the notes and designs of Larry Trask and Bolivar Trask, his father and creator of the original Mk I Sentinels.
Lang also went to the Council of the Chosen Ones, part of the Hellfire Club, who provided him with funding to create new Sentinels and an orbital HQ. The Council also ignored that Lang wanted to destroy every mutant he could capture. When Lang's old friend Michael Rossi discovered Lang's true intentions and threatened to shut Project Armaggedon down, Lang had the plane carrying Rossi back to Washington D.C. destroyed.
The Mk III Sentinels were unfortunately inferior to the previous ones because Lang worked with only incomplete notes from the Trasks and because the secret of his activities stopped him from hiring enough experts. He also made them less intelligent than the previous Sentinels to stop any possible rebellion. Notably, Lang built Sentinels that looked like the original X-Men, which became known as the X-Sentinels. The X-Sentinels confused the newer X-Men to such a degree that they were ending up thrashed, but then Wolverine discovered that these impostors were not human and ripped up the Jean Grey lookalike, exposing their true nature.
Lang's Sentinels and space station were destroyed in a battle between the Sentinels and the X-Men. Lang was lobotomized during the battle, but he had made a backup of his mind patterns that was used as a base for the mind of Master Mold and his other Sentinels.
According to Stuart Vandal, "[his] correct name is Steven Lang. Stephen Lang is incorrect and just like the misspelling Moira MacTaggart (instead of the correct Moira MacTaggert) ofted used incorrectly." Indeed, "Stephen" is commonly used alongside "Steven".
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