Thats easy. I joined a guild when I first got to the server and made a warrior. I played it for maybe 2 weeks and then then dumped him for my paladin. I played that paladin for a month or so and made my mage really just so I could stop walking to and from events.
My mage blew up. He was a low ranking dalaran senator, basically a paper pusher. I dont remember what committee i had him on since it was just filler but it amounted to making sure there wasnt excessive trash on the streets and the amount of weight was evenly distributed around the city based on a dubious study that showed the place risked flipping over in high wind. Literal nothing work.
Anyway, he joined up and (as it was a magical themed guild but with only loose ties to dalaran) he was very much your sort of uppity haughty mage-lord. Critiquing every incident of non standard magical practice, conjurings performed without filing paperwork and, this is the part that got big, constantly in a state of alarm at the presence of warlocks.
Anyway, i had never meant for him to be part of anything major, just your run of the mill character that was perhaps a little antagonistic but not much else. Before I knew it though people had latched on. The officers made an announcement warning people to be wary around him and people who just generally were annoyed by him began to make rude comments so we spun that out so he interpreted it all as suspicious behavior.
Before long he was poking around, asking questions and there was general unease. The guild lore explained that they had very limited approval to use fel magic but no demon summoning and it became a sort of subplot that both sides suspected the other of trying to frame the other.
I wont bore you with all the details but over the next year he went from nobody to a major guild villain with multiple subplots revolving around him, he slowly turned the gms son against them by playing off his desire to learn more magic and it culminated in a big confrontation at his office balcony in Dalaran after he and my mage vanished through a portal. The GM and he were in the middle of negotiating a resolution (in short, he would step down and my character informs the senate he was mistaken about the presence of illicit activity) when we suddenly hit our guilds peak hours. As we were having this meeting in secret they saw the two of us in Dalaran and ic pinged the gms commstone. Suddenly they flood to the city thinking the gm had been arrested and it turns into a wild standoff as before the metaphorical handshake they burst into his office making demands. Immediately my mage is back to posturing and it all goes off the rails.
One of our guild mates, another officer was mostly inactive but had gone through our guilds very strict rules for icly being recognized as an archmage and was serving as my mages defacto supervisor on the issue since the guild was technically under the officers charter. It concludes with the gm announcing he was turning the guild icly to his inexperienced wife (a warlock as well but not public) and that the archmage had resolved the investigation bh having them evicted from the tower. The last scene was my character invoking a last minute bit of fluff allowing the upper crust of dalaran to extend a special offer to anyone with magical talent inviting them go study in the city academy. He hands the letter to the gms son and effectively steals him away from his pseudo mother figure breaking her heart, a sort of last jab in order to save face.
It was glorious.
For years after that he would still be mentioned by characters in the guild, cursed as a monster. It also gave the guild a springboard to restructure and take on a less school focused and more action geared rp style. I think the reason it stands out so much to me was that none of it was planned. The officers mostly talked on ventrilo back then and I didnt have a working mic (i didnt use mine much so when it got chewed on by the dog I never bothered to replace it.) So it really was this sort of epic adventure thaf was unpredictable because neither side was coordinating or plotting with each other just acting and reacting naturally. It was great fun for me and I think the officers enjoyed having a plot that they could be a part of where it was open to everyone, they could build things off of and didnt have to closely follow every step or try to stick it to a certain plot path.