Ok ! ive been in Brill several times now. Either im just unlucky or its bugged in Brill.
This is from wowhead about the quest:
Which you get will depend on whether the village is currently under attack from a fire-flinging [Shade of the Horseman]. If there are no fires present at that time, you’ll get Let The Fires Come! If the village is under attack when you arrive, you’ll get Stop the Fires! It’s essentially the same quest: help put the fires out no matter when they arrive, and loot the giant pumpkin at the end.
Edit:
Now i finally got the see the houses on fire in Brill !
We were 3 ppl throwing water on the fires but the fires never stopped completely !
How many ppl do u have to be throwing water at the same time to make it work ?
It might also be that the fires are bugged in Brill…
Most likely it spawns if you get the daily where you should save a village from fire.
I think that it works in that way because I ran to Goldshire and happened on the horseman. I tried to put out fires, and then after a few minutes I took this quest and he came running again and set everything on fire