Alright I’ve finally had some time to sit down between classes and hammer out some of my thoughts for you. I’ll try to go through the prompt as you’ve written it and offer my critique, as well as some more general advice on how to GM well. Full disclosure: I write and GM in the [i]Advanced section.[/i] This will subsequently color the entirety of my feedback. Some of this will not apply 1:1 and some of it may feel a bit harsh, so take things with a grain of salt.

[indent][indent][color=e2e2e2]. . . a coven of mages accidentially destroyed one of the worlds greatest cities . . .[/color][/indent][/indent]
[i]One does not simply ...[/i] I don’t think this trigger event works for creating a believable premise. I think instead you shouldn’t be afraid to invoke a fanatical cult or something more malevolent here. If the church that later becomes this holy inquisition has something to very legitimately call a threat, then nobody questions their relentless persecution of magic. You want the obvious thought process to be “All magic is temptation. Temptation leads to misuse. Misuse is dangerous.” or similar. Equating magical power to existential danger ensures widespread support and minimal resistance to what equates to genocide.

[indent][indent][color=e2e2e2]The religion preached that the great god of life was angered by the destruction of his favored city that was destroyed by magical fire.[/color][/indent][/indent]
Similar thing, though this one has a shred of subjectivity coming through with it. I think it reads more believably if written as “God says ‘thou shalt not give in to temptation.’ Magic is temptation. Magic is a sin unto the lord.” or something of that sort. This is sort of the natural evolution of the previous, enshrining the idea into faith rather than reason even further cements the blind adherence you want for believability of setting.

I think if you take these two points (or rather facets of the same singular point) and apply them thoroughly through the concept, you’ll build a stronger premise for it. Beyond this, further worldbuilding (importantly including the construction of the magic system in play) is something I cannot reliably consult on without us sitting down for an extended period of time working through things.

[hr]
As for running a group, I can offer some basic guidance for now.

[b]Have a plan.[/b]
[indent]A general story arc along which to keep things moving helps a lot. I know many people like freeform, and this isn’t to say you can’t allow players to have their own stories to tell, but having a very general, overall arc that you can use to keep things moving in a set direction has helped me keep things moving when without it, they would have stagnated long ago.[/indent]
[b]Have standards.[/b]
[indent]Particularly at the casual level, I see all too often GMs make rules, but then fail to enforce them in any meaningful capacity. I admit it’s no fun being the bad guy, but you’ll be more upset at yourself if you do nothing, than if you do something. Stick to your guns. Don’t be afraid to kick people if it becomes necessary. Don’t be afraid to deny character sheets if there’s something you don’t feel fits the setting or is overpowered.[/indent]
[b]Fully build the idea before recruiting.[/b]
[indent]At least to 90% or so is good. The interest check you post should be pretty close to the main OOC thread you’ll post later. If you don’t (and again I see lots of GMs do this), the time delay involved in finishing it often causes player interest to fade and you lose half or more of your interested players. Sure, it sucks building an idea that then doesn’t take off, but when the interest starts rolling in, you’ll be glad you did this.[/indent]
[b]Demand character sheets at the interest check.[/b]
[indent]Some people put the CS template up, but surprisingly few actually demand them. “I’m interested” doesn’t count for shit when judging who will or will not commit to an RP. A measly 50% player retention going from interest check to main thread is not uncommon. But if you judge your interest by completed (or at least partially completed) character sheets, 70% or higher retention becomes the case.[/indent]

This post has rambled on long enough so I leave you with this as my wisdom for now, as I have class again in 20 minutes. PM me, or find me on discord (you are in the discord for Congee’s romance RP right?) so we can keep going back and forth if you’d like, as I’d rather not clog up this thread too much more.