(covers information from several alternate timelines)
Doctor Tolian Soran was an El-Aurian scientist who, in the 24th century, attempted to destroy two stars in an effort to gain entrance to the Nexus, an extra-dimensional realm in which all one's desires are transformed into reality. In the process, he was indirectly responsible for the death of James T. Kirk.
Life before the Nexus
Soran was born before 2071. He had a wife, Leandra, and several children. His entire family was killed when the El-Aurian homeworld was destroyed by the Borg in the 23rd century, though Soran himself escaped. The traumatic loss transformed Soran from someone who "wouldn't hurt a fly" to a profoundly cynical and nihilistic man who believed that death was the sole constant in the universe.
In 2293, Soran was among the El-Aurian refugees being transported to Earth aboard the SS Lakul. Outside the Sol system, the Lakul was caught in an extremely intense gravimetric distortion trailing from an energy ribbon which acted as an entrance to the Nexus.
While trapped in the distortion, Soran and the other passengers experienced brief moments in the Nexus, where Soran was able to live a life in which the Borg had not killed his family. Soran's experiences in the Nexus were short-lived, however, as he and the others were beamed away by Captain Montgomery Scott to the USS Enterprise-B, which had responded to a distress call. Upon finding himself in the Enterprise's sickbay, Soran began ranting, demanding to be returned, forcing a nearby Commander Pavel Chekov to sedate him with a hypospray. Soran subsequently became obsessed with returning to the Nexus and dedicated the next seventy-eight years of his life to discovering a means by which to do so.
By the year 2371, Soran had developed a plan whereby he would destroy two stars, thereby altering the gravitational forces near the energy ribbon, altering its course and bringing it to the planet Veridian III, where Soran would be waiting for it. To accomplish his goal, Soran entered into an alliance with the Duras sisters, who agreed to secure trilithium, a nuclear inhibitor with which Soran would destroy the stars, in exchange for research allowing the sisters to develop a trilithium-based weapon.
As Soran prepared to fire a solar probe equipped with trilithium into the Amargosa star from the Amargosa observatory, the outpost came under attack by Romulans who were attempting to retrieve the trilithium, which the Duras sisters had stolen from them. Soran was rescued by the crew of the USS Enterprise-D, and was later able to return to the observatory and launch the probe, destroying the star.
As the Enterprise crew attempted to rescue an away team from the observatory before the resultant shock wave arrived, Soran was retrieved by the Duras sisters and subsequently traveled to the Veridian system, where he intended to destroy the Veridian star. The Enterprise crew ultimately uncovered Soran's plot, realizing that Soran intended to destroy the Veridian star despite the fact that it would destroy all planets in the system, including Veridian IV, which supported a humanoid society of 230 million individuals.
The Enterprise arrived at Veridian III, where Captain Picard beamed to the surface in an attempt to reason with Soran. Picard was ultimately unsuccessful, however, failing both to appeal to Soran's reasoning – Soran grimly stating that everyone was fated to die and that he was no longer concerned about preserving life – and in his subsequent attempt to penetrate the force field around Soran's equipment and stop him from launching the probe into the star. As the star was destroyed, the energy ribbon was diverted to the surface, where it transported both Soran and Picard into the Nexus, immediately before the destruction of all of the planets in the system and the Enterprise itself.
Inside the Nexus, however, Picard was able to make contact with James T. Kirk, who had been pulled in during the Enterprise-B's encounter with the ribbon. Picard managed to convince Kirk to leave the Nexus with him, traveling back in time to a point before Soran had launched the probe.
As Picard attempted to sabotage Soran's launcher, Kirk fought Soran, a confrontation which ultimately led to Kirk's death, when he fell off the scaffolding holding Soran's equipment while trying to recover the remote control for the launcher. Kirk's sacrifice, however, allowed Picard to successfully disable Soran's launcher. As Soran returned to the probe, he found that Picard had engaged the locking clamps, destroying the launcher and killing Soran in a massive explosion.
In the aftermath, Picard reflected that he disagreed with Soran's perception of time as a predator, preferring to see it as a teacher that encouraged him to treasure every moment as it would never come again. (Star Trek Generations)
In an alternate timeline, Soran had originally succeeded in reaching the Nexus. Since Soran's plan initially worked, it is suggested that Soran is in fact still in the Nexus, since time has "no meaning" there. (Star Trek Generations)
Soran was played by actor Malcolm McDowell.
In early drafts of the script, the character was named "Moresh". This was changed as it unintentionally resembled the name of cult leader David Koresh. (Star Trek: The Next Generation Companion (2nd ed., p. 319))
At least two different versions of Soran's death were written for the script of Star Trek Generations. Additionally, two versions of Kirk's death were shot. The first involved Soran directly killing Kirk with a disruptor bolt, shooting him in the back. This version proved unpopular at early screenings so an alternate death (the scaffold bridge death) for Kirk was filmed and used in the theatrical release.
While Paramount was casting Star Trek Generations, Rick Berman received a call which notified him Marlon Brando was interested in portraying the role of Soran. Berman was overjoyed and highly excited by the news. "And I went to see the president of the studio," Berman continued, "and I said, 'Marlon Brando wants to play Soran.' And the response was, 'What does he want?' And I remember the president of the studio just laughing at me, and that was the end of that." When Berman relayed this sequence of events to Brannon Braga years later, Braga remarked that he thought casting Brando as Soran would not have been beneficial for the film. ("In Conversation: Rick Berman and Brannon Braga", ENT Season 1 Blu-ray special features)
When Tolian says "They say 'time is the fire in which we burn" he is in fact quoting the poem "Calmly We Walk Through This April's Day" (also known as "For Rhoda") by Delmore Schwartz. The full poem can be read here. According to Ron D. Moore, the writing staff found the quote in Bartlett's Familiar Quotations. (AOL chat, 1997)
The early edition of the novelization of Star Trek Generations featured one of the alternate death scenes for Soran. In this version, Picard succeeds in disarming Soran's probe, preventing its launch and the destruction of the star, and allowing the ribbon to pass the planet by. Soran charges at Picard in a fury, who snatches up Soran's disruptor and shoots him dead in the chest. The novelization also notes that Soran was aware of Picard's trip into the Nexus, due to the lack of alternative explanations for Kirk's presence, increasing Soran's rage at Picard for depriving him of his wife again.