I think Raverna is more of a lone wolf. How she got there is unclear, and pretty much up to the DM, but what the book has to say in the obvious areas about her:
"Though she has not officially allied herself with the Children of the Inner Light, Raverna met their leader and took it upon herself to rid the REgion of the evil fiends haunting it. Unless the PCs are evil, she views them as potential allies."
Mind you, that's from her Random Encounter text on page 350. Most of her other Random Encounter texts are fairly similar. And, frankly, that's pretty much all the information that the book gives on her. I get the feeling she's there as a bit of a DM's tool - much like the random encounters which can bring either Besar (bralani Ftr3, I believe) or one of the inevitables down to the south end of Region E to pull a party out of bad shadow fights if needed.
So, Raverna could be there for whatever reason you desire, but she's not connected to the garrison. Think of it as 'the enemy of the enemy is my friend.'
In terms of powerful artifacts, there's a +3 dragonbane dagger in C72 under a big pile of rubble that could easily have more fluff attached to it, but I haven't been able to find info about a powerful evil sword in C in my scan through the region. However, I did remember other interesting artifacts...
Region D has Truesilver, which is interesting enough on its own. (Page 175)
Region E has a
nine lives stealer, but it's been discharged.
Region F, I haven't reread recently, and can't tell you much about.
Region G has some pretty vile stuff that's being kept locked up by the celestials, such as a
talisman of ultimate evil, and the
spear of Ternius (a cool weapon - +4 humanbane longspear of wounding, with purple worm venom attached. Shame the celestials won't let anyone use/touch it.). Lord Tarnaticus has an interesting weapon with a story to go along with it. (Page 361) And I almost forgot - a very powerful item with a very powerful guardian, the
rod of tainted earth, with a fiendish elder earth elemental (24 HD!) guarding it.
P.S., Hussar, if you don't use this against your party, I'll be sad.
Region H has
Lyonatar's Eye (page 395), which is pretty much damn impossible to use/activate with things as they are in there. Jolinaar has some cool stuff, though. (page 427)
Region I, as previously mentioned, has Eight Strike Fang (page 435), among other things. It's also got the Ritual of Unmaking, but don't expect your spellcasters to start tossing that one around.
Region J has the
chaos diamond in the possession of some behirs (page 508). There's also
Baltazzar's Lantern (a variant djinni's lamp) (page 516). On the same page is a gem that acts as a
brazier of commanding fire elementals with some advantages and drawbacks. The gem radiates evil, though, as it summons an evil elemental. Shame. There is, of course, Tyrus' hoard, but good luck getting that stuff.
Region K has the
crystal of vile attraction (page 543), a cursed item. There's all the tanaa'ryl items (page 557), which have a couple of drawbacks. There's a
gem of lightning, well hidden. (page 583)
Region L has the watrazor. Enough said. The whole region's effectively built around that - completing it will be an epic challenge. (And whoever built Mahg'Gog has succeeded in impressing me. Giving a kraken wings? Awesome!)
Region M I haven't read in forever. No idea here. Didn't impress me that much.
Region N... now we get interesting. This is where that powerful evil sword resides...
Varnarul, a CE +5 celestialbane keen bastard sword. (page 767) Besides that, there's so much STUFF in this region that I can't cover it all. Let's put it this way - it's the old lost treasures of the ages.
Region O... geez, you won't get here for a while.
That's pretty much all of the major treasures and artifacts. Feel free to add more.
Edit: Clearly Hussar and I agree on the usage of Raverna.

And I can't believe I just went through the whole WLD book and spat out all that information. Pretty amusing.