Stryker developed the serum from harvesting his son's spinal fluid, which was so powerful that it acted as a mind controlling agent capable of subjugating mutants to do his bidding.
When Stryker uses the serum on mutants, they develop white rings around their eyes, signifying that they are under Stryker's control. In addition to this, a reddish circle appears on the nape of their neck, a side-effect of Stryker administering the serum to control them.
Styrker captures Nightcrawler and brings him to his secret facility at Alkali Lake, wherein he subjects the teleporting mutant to Jason's serum. Stryker instructed Nightcrawler to assassinate President McKenna, which would enable him to frame Charles Xavier for the attack and have full access to the X-Mansion. Following Erik Lehnsherr's incarceration in his plastic prison after the Liberty Island incident, William Stryker made several visits to Magneto to interrogate him, and through using Jason's serum on him he gained an intimate knowledge of Magneto's past with Charles Xavier, and the existence of Cerebro.
When Charles came to visit Erik in the aftermath of the attempted assassination, Stryker filtered a gas into the chamber that rendered Charles unconscious and sent his subordinate Yuriko Oyama to capture Cyclops. Once they were subdued, Stryker brought them both to Alkali Lake, where he subjected Scott to Jason's mind control serum, using him to attack his lover Jean Grey against his will. With Xavier, however, his powers could resist the potency of Jason's serum, so Stryker brought Jason himself to subdue Charles personally, and forced him to operate Cerebro and eliminate all of the mutants worldwide.