Rary's Story, at least...
Well, Vecna I can’t really help you with (cannonfire.com probably can though), but here is a piece from Rary the Traitor that can explain some of it:
Rary's Tale
Rary the Mage did not initially seem the sort of individual who would betray his friends and become one of the most reviled men on the continent. He was once described as “quiet, dignified… a skilled mediator and peacemaker,” and a man whose gentle nature prevented him from leading the Circle of Eight, or even challenging its other members, so Rary's descent into the realm of evil was indeed puzzling.
The Dignified Mage
Although well into his seventies at the beginning of the Greyhawk conflict, Rary retained a youthful outlook and energy, and his primary motivation seemed to be learning, rather than the pursuit of power. Archmage of Ket, Rary was well known for his skill at resolving conflicts in the complex atmosphere of the Ketish court. His friendship with the Paynim nomads, while considered something of an eccentricity within Ket, actually helped the mage prevent Paynim raids into Ket, or at least helped reduce their severity.
One of the most baffling aspects of Rary's treachery is that at the time the war broke out, he was largely retired from the Circle of Eight, content to dwell quietly in his tower at Lopolla, pursuing his hobbies and avoiding confrontation. Rary attended meetings only when issues of special importance were discussed, and even then seemed reluctant to directly confront such volatile individuals as Mordenkainen and especially Otiluke, who is said to have harbored a special dislike for the Ketite.
In truth, Rary's retirement was more a result of his frustration with the Circle of Eight and its contentious ways than a genuine desire for retirement. As time passed and Rary's gentle approach worked less and less, he began to spend more time in his tower or wandering the plains mounted on his magical destrier. In this guise, he was known to the Paynims as only “The Rider.” As he traveled among them he learned many things from their shamans and chieftains, and his gentle nature began to change.
Surely, Rary maintained his calm, reasoned exterior at those meetings which he did attend (further infuriating Otiluke in the process), and his peacemaking efforts in Ket continued unabated. But the process of Rary's conversion had begun, fueled by the twin fires of his own frustration and the violent influence of the Paynims.
The change in Rary's nature was not a malevolent one. He could just as easily have continued as a powerful advocate for balance and neutrality or even good, had he not begun to study the histories of the demi-god Iuz, the Scarlet Brotherhood, and the Overkings of Aerdi. As more time passed, Rary's admiration of these evil forces grew. Here, he realized, were individuals who knew the value of force, who shunned the pointless squabbling of the Circle of Eight and other lesser powers. Of course, Rary knew mistakes were sometimes made. The Overkings were now hopelessly mad and mired in decadence, Iuz's violence had little direction or purpose except his own aggrandizement, and the Scarlet Brotherhood's obsession with secrecy did little to advance its cause.
Still, Rary realized there were things to be learned from these individuals. If he could control the base urges and character flaws that had led these others astray, perhaps he could achieve greatness, and even surpass the exploits of those in the past. With the Flanaess, or at least a large portion of it, under his control, he could bring change and overcome the meaningless bickering of the Circle of Eight. This enormous change in Rary's temperament took place slowly. The great mage's outward calm and good nature belied a storm growing in the depths of his soul.
War and Treachery
Rary's admiration for Iuz and the Brotherhood grew during the war. As other members of the Circle worked frantically behind the scenes to head off the conflict and preserve what civilization remained in Greyhawk, Rary's mind turned more and more to thoughts of evil. He researched long-forbidden spells, studied the histories of ancient conquerors, and learned more from his Paynim friends as corruption and wickedness slowly crept into his heart.
During a particularly unproductive session of the Circle, Rary quietly withdrew in the face of Otiluke's bluster and returned to his tower in Ket. There, brooding upon his decades of ceaseless toil and frustration and his lack of success in the path of neutrality, Rary finally and irrevocably fell under evil's sway. Returning to Greyhawk, Rary enlisted the aid of Lord Robilar, a powerful if somewhat unstable nobleman with a substantial household guard, and together the two formulated a plan to put themselves into a position of power in the Flanaess.
Diplomats and high officials from all across the continent would soon arrive in Greyhawk for the Great Signing. Using his access to the Great Hall, Rary planned a series of magical traps which would destroy everyone in the building, including diplomats, nobles, the Lord Mayor and his staff, several guildmasters, at least half of the Circle of Eight, and the detested Otiluke. That done, Rary would assume control of the Circle. As an emergency measure, Rary would dispatch agents to those countries whose diplomats had been slain. These diplomats would gain the confidence of kings and chieftains, eventually giving Rary access to and control over numerous nations. The killings themselves would be conveniently blamed on the Scarlet Brotherhood, which had made itself unpopular during the war.
Death in the Great Hall
Unfortunately for Rary and Robilar, as Rary finished setting his magical traps, several members of the Circle entered the hall for a last-minute inspection of the site before the signing. Caught red-handed, Rary first at tempted to flee. Forced at last into open conflict, he attacked with a fury born of years of pent-up anger. He set off the still incomplete magical traps. Tenser fell first, caught by surprise. Otiluke struck back savagely, wounding Rary and forcing him back.
Instead of counter-attacking Otiluke directly, Rary set off several more traps, injuring Bigby, who had just joined the fight. Overcome with concern for his friend, Otiluke's concentration broke, and suddenly Rary's spells had him, rending and burning him at once. Abruptly all the remaining traps went off, plunging the hall into noise, fire, smoke, and the lambent glow of magic. As the smoke cleared, amid the crash of falling masonry and the tinkle of broken glass, Bigby, himself badly wounded, crawled over to see to his friends, only to find both of them slain beyond hope of resurrection, and Rary had vanished without a trace.
In the ensuing confusion and shock, Lord Robilar's own troops struck, destroying every one of the dead wizards' hidden clones, assuring the permanent death of both Tenser and Otiluke. Within hours, Greyhawk warriors had occupied Robilar's citadel and began to search for the pair, but it was too late. Faced with the collapse of their plot, Rary and Robilar fled, along with those troops loyal to them, and no one knew where.
A New Kingdom
In the weeks that followed, as Greyhawk slowly recovered from its shock, Rary was condemned from every quarter. Expeditions scoured the vicinity of the city, searching for Rary and his co-conspirator. A diplomatic mission to Ket returned with the astonishing news that not only was Rary missing, but his entire tower, which had so long graced Lopolla's sky line, had abruptly and inexplicably vanished overnight. Rumors began to circulate that Rary had fled the Flanaess and possibly Oerth altogether.
These rumors did not last long, as search parties heard disquieting reports from the Bright Desert. The Abbor Alz barbarians, who sometimes traded with the desert nomads, reported that a mysterious tower had appeared in the Brass Hills in the heart of the desert, and that the master of this tower had proclaimed himself “Monarch of the Bright Lands.” Many nomads, it was reported, had fallen under his sway, and those who had not were swiftly persecuted and pressured to do so. The desert centaurs, previously known only by rumor and second-hand reports, were said to be resisting the new rulers, but their struggle seemed doomed. The new king was said to be a mage of unsurpassed skill, and had a host of strange (some said unearthly) warriors at his command. Clearly, Rary the Traitor had at last appeared.
I hope this helps