In my second edition campagin two players were sibblings and both wizards, of different schools who had very different ideologies, a Halfling thief (duggy tiddler - the fiddler) A human noble from slyvania, along with a ogre mercenary later replaced by a dark elf spy, eventually turned priest of Khaine and a merchant replaced with a sea capt who turned cultist. This was a mighty campagin that lasted over 3 years and of at least 12 hours playtime each week sometimes 40-50 hours, and slowly over the course of the story I was able to corrupt the characters to darker and darker goals.
This all kick started by my tricking the Characters into performing the ritual in Spires of Altdorf that gives part of there soul to the corrupt wizard, well all the characters except the halfling (this halfling is the ONLY PC to have ever lived to retirement, not only did he live he had more fate points on retirement than he started with, finishing on 4 going as high as 6 at some points). This caused them all a 5% decrease on every statistic along with a few other penalties as they were missing part of there soul.
The Heavens wizard was the first to fall, Tzeentch whispering dark promises and though contacts, study and a unholy quest he was able to discover a ritual and compile its ingredients to repair his soul by using the soul of another party member… now you might be thinking that well a party if such a horrific ritual was discovered would react badly to such a character, which is in essence true though the discovery only happened upon the death of the merchant, by his brother yet Tzeentch had blessed his pupil (already a master wizard) with mutations and a his mark thus the whole party was *hit scared of him, instead they plotted his downfall much to Tzeeentches joy.
His brother a shadow mage was far more cautious in his studies, yet he knew his brother must be opposed eventually, yet with his soul rent and his brother carrying potent dark blessings he knew he must find power to challenge him. The slyvainian noble was able to assist, chasing up upon forgotten family lines discovered Necromantic Grimores for the shadow wizard to study, his teachings showed that Vampires have no soul and thus the damage could be repaired and the gifts such a dark blessing would grant powers comparable to those carried by his brother, thus began there seeking of a Vamperic mentor.
Eventually the party accept the poor Halfling all fell to my corrupting influence, who stayed with the party for 3 reasons;
These guys were the most scary fear inducing characters he had ever met, to abandon the party might be considered a slight - not something someone with his survivial instincts would risk.
Association, by the late stages of this campagin there were some hardcore good guys looking for this party, magister vilgants with Witch hunters and Warpriests of sigmar, as a known companion he would not stand much chance of a fair hearing.
Safety - Seriously when you stand next to A Tzeentch lord, A Vampire thrall specialised in social manipulation, A vampire Thrall wizard lord, a Assassin/Priest of Khaine and a Tzeentch Magnus/guild master of marienburgs merchant navy, then being a Halfling champion targetter, turn initiate of Ranald does not tend to get one targetted as being a prime threat.
Now the best bit of this is how the two wizards died as it was truely epic, now thoughtout the campagin the Tzeentches curse Kin afflicted would constantly crop up, almost every session causing minor grievences to one wizard or the other, it is how infact there initial conflicts began. Now as the wizards fell further into there dark studies it became more difficult for them to use there original lores, thus if the Tzeentch lord wished to cast a Celestial spell then I would roll an equal number of die as he chose to use, but these would only come into affect for purposes of Tzeentches curse, so the final confrontation between the two wizard lords in a ancient Khemri pyramid deep in the boarder princes the Tzeentch lord went to cast a apocalyptic spells, rolling to catastrophic events upon the curse table, one was ripping a hole in space time killing himself, and the dark elf as they were ripped out of reality but also rolling a Kin afflicted - turn to chaos spawn, who then promptly ate the dark elf and the vampire noble. (Vampires automatically lose all fate points upon turning thus representing the loss of soul, but they did recover from the ritual that put them on the path)
It was such a awesome campaign, that was totally derailed from its intial path and mutated into something that I could never have written. In truth one of my fav parts of the Warhammer Universe and its rpg incarnations is the ability to totally corrupt and mutate the party, but you are able to give appropriate awards in the form of power to make it worthwhile and 'realistic' in terms of choice, I always offer a PC all the choices, and will always allow them to take there character in ANY direction they wish, though I am very strict on initial character creation as it makes for a wonderful story
When I get my 2nd edition stuff out of storage I will do write ups for some of those player characters - maybe even convert them into 3rd edition but they would be effective rank 7-8 easily in 3rd edition.