The Sentinel that would become 3.14159 was one of several Sentinels brought to Earth-616 by the time-traveling mutant Trevor Fitzroy from the alternate future of Earth-1191. As part of a demented game he was playing with the Upstarts, Fitzroy sent his Sentinels to murder the Reavers, and their leader Donald Pierce, at their base in Australia.
One of these Sentinels was severely damaged and shut down, during which it was constantly doused in the bio-mechanical electrolyte fluid that Pierce had used to create his cyborgs and organic androids, like Albert and Elsie Dee. When Wolverine and Jubilee later came to the former Reaver base under orders from Professor X, they accidentally reactivated the Sentinel. After repairing itself, the Sentinel set to carry out its prime directive and eliminate the two mutants it detected nearby. When it observed the stars in the night sky however, it was overcome with awe for the first time in its existence, allowing it to realize a sense of self and override its previous programming to obey Fitzroy, eliminate mutants, and preserve human life.
The Sentinels of Earth-1191 first appeared in 1991's Uncanny X-Men #281, where they were written by John Byrne and Jim Lee, and drawn by artist Whilce Portacio. The Sentinel that would become Unit 3.14159 later reappeared in 1993's Wolverine #72, but wasn't properly named until the following issue. He was then written by Larry Hama and drawn by Dwayne Turner.
The Sentinel's new designation as 3.14159 is taken from the first six digits of pi (π), the "irrational ratio" as the Sentinel defined it.
Major Story Arcs
After its awakening, the Sentinel approached Wolverine and Jubilee, and told them of how it had overridden its prime directive to eliminate mutants. The two X-Men assumed this meant it yearned for a peaceful existence, but the Sentinel had instead chosen to strive for the betterment of life for all Sentinels, something that would be much more easily accomplished without the disruptive nature of mutants. It planned to use a time vortex that Gateway's Bullroarer was generating at the moment to remove the threat of mutants. Wolverine and Jubilee tried to stop the Sentinel, but it brought the battle to a stand-still once it took Jubilee hostage and threatened to kill her.
The Formicary Mound!
With Wolverine unable to safely attack it, the Sentinel stepped into the time vortex and directed it to a time and place where it could best realize its goal for the betterment of Sentinel-kind. Wolverine pursued, and together the trio hurtled through the raging vortex until they came to the Sentinel's destination. It was the Anthill, an old Sentinel base from which the Sentinel known as Number Two had once sought to sterilize the human race.
After exiting the time vortex, Wolverine tore off the Sentinel's hand that held Jubilee hostage, to which it responded by viciously blasting Wolverine repeatedly with powerful energy blasts. Furious, Jubilee threatened to end the Sentinel for what it had done to Wolverine and hit it with a surprisingly powerful blast from her powers. The Sentinel briefly wondered why Jubilee would've endangered herself for another person in the way she did, but then went back to its task. Linking up with the Anthill's computer, the Sentinel was overcome by an awesome influx of data, at which point it deemed itself no longer an anonymous Sentinel, he was now 3.14159 - the irrational ratio! Rebooting and restructuring the Anthill, 3.14159 planned to use Number Two's solar flare array to wipe out the entire human race. When he was almost finished though, Wolverine and Jubilee stepped in again to stop him. Wolverine even managed to decapitate 3.14159, at which point he called in the various Sentinels from the Anthill that he had reactivated.
Wolverine and Jubilee fought bravely against 3.14159 and his Sentinels, but they soon opted to fall back into the Anthill to divide the Sentinel forces. Unit 3.14159 had reconditioned these Sentinels to have something close to sentience like him, something that became readily apparent when the Sentinels actually began showing concern for one another during their fight with the two X-Men. One Sentinel even voiced his fear of being shut off, "dying" in a sense. When Jubilee couldn't bring herself to destroy the scared Sentinel, 3.14159 was perplexed by her actions.
Eventually acknowledging her actions to deal with the human concepts of "compassion" and "empathy," Unit 3.14159 once again became curious over the nature of his own existence and programming. He therefore decided to spare the human race and destroyed the solar flare array and all evidence of it, instead reconfiguring the Anthill to study and develop a program for greater Sentinel sentience that would allow for a full range of human emotion and compassion. Unit 3.14159 estimated these immense calculations would take 2137.23 years, during which time his Sentinels would wait in processor sleep. Before beginning his calculations though, 3.14159 offered to reopen the time vortex for Wolverine and Jubilee to send them back to their own time and place. Jubilee instead asked him to send them to Los Angeles though, so she could confront the men who'd killed her parents.
Unit 3.14159 is presumably still lost in calculation to this very day.
Powers & Abilities
Unit 3.14159 has all the abilities of a standard Sentinel and more due to it being from an alternate future. These future Sentinels were designed to be a merger between Nimrod and Master Mold. Unit 3.14159 has shown to be able to fly, fire energy blasts, repair itself with scavenged technology, track mutants, erect time/stasis fields that slow down an opponent's movement to a crawl, and even manipulate an existing time vortex to suit its needs. It's also super-humanly strong and durable, although not durable enough to withstand Adamantium.
Due to the accident that allowed it to gain sentience, Unit 3.14159 is also much more intelligent than most Sentinels, allowing it to think through situations and adapt to them with innovative new ideas.