Wychmire Swamp in Caledon Forest
Falias Thorp, but it's very frequently contested. If it's not available next-best is probably Caledon Haven.
Is this going to take long? These mosquitoes are as big as basketballs!
Could be a while. You have to wait for a cabbage called Gamarien to decide he feels like wandering around the swamp and he only gets the itch once every couple of hours.
So what is there to do while I wait?
Fight the Nightmare Court. Over and over. They want Falias Thorp so badly they send eight waves of grunts to take it every ten minutes or so. Sometimes they even win, which can lead to a nasty surprise if you logged out there thinking it was safe.
Other than that, the area's a bit short on entertainment. It's a swamp.
How will I know when it starts?
A big orange blob appears over Gamarien's head. That's a bit of a giveaway. Also the meta-event text changes to some variant of "Shadows grow long in the heart of the swamp". Spooky! Gamarien does speak but only to ponder to himself not broadcast to the whole map so you'll only hear him if you're quite close.
And my role in all this?
Guess. Your job is to follow Gamarien and make sure he doesn't get eaten while he strolls about orating portentously. There are a number of stages, all of which end in mass slaughter as you prevent a range of wildlife, locals and mindless undead from doing what you'd so dearly like to be able to do yourself, namely shut Gamarien up for good.
That sounds like a lot of walking.
Gamarien walks. You'd better run. There are several discrete events, each of which gives karma and xp but they pop very quickly, as do the things you have to kill, which all seem to have very few hit-points. Unlike The Maw, the terrain mitigates against your getting to all of them in time. Pick a target and stick to it; if you tear around trying to get all of them you'll most likely get none. And when the Wurm appears, don't stand there gawping. He dies as fast as everything else.
Or you could avoid all the running bout and go wait at the corpse of the dead worm from last time. You can see it from where Gamarien starts. He just walks in a big circle.
Definitely and not just for the rewards. The Wurm when it appears is spectacular but if there's anything like a crowd waiting he'll be dead before you can really appreciate him. He stands repeated viewings. Apart from the frankly ludicrous early stage when Gamarien and the Nightmare Court determinedly run through their lines in direct contradiction of the reality around them, the whole thing is fast, furious, frenetic and fun.