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Revenant I always recommend first since for whatever reason the person who made the module decided not to allow voting on it, meaning it wont be on the 'top' lists for NWN vault and will only show up under searches or links like the one I provide. Why they did this I don't know because the module is
fantastic. Probably one of the most unique and well executed premise for a module or even rpg period. You are a revenant, an undead monstrosity out to avenge your own murder. Whether you choose to be the monster many in the game will treat your character as or something else entirely up to you, some great roleplaying opportunity here and tons of replayability. Great module!
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Tales of Arterra and
Tales of Arterra The Awakening are a much more traditional rpg experience but also really well done. Essentially it's what you might have hoped and expected the OC would be in the first place. Not the most inventive module as its pretty heavily influenced by other rpg in terms of story, namely Baldur's Gate and Planescape: Torment, but that's hardly a bad thing and it's done quite well here. Good length too. Like I said, this is more like one would have hoped the OC would have been to begin with. I liked the characters, I enjoyed the story, gameplay was entertaining, can't really complain.
-These two are more like Revenant however in that they approach things a bit differently.
Elegia Eternum is a pretty short but interesting module on its own, but its the sequel
Excrucio Eternum that just goes nuts and makes this little series so memorable. Again, short module, but it packs a damned good punch for its length and is definitely unlike most rpg experience you would fine elsewhere. Extremely heavily story based so you wont get much in the way of combat scenerio here as that's not really the main focus, but the story is more than interesting enough to make it worth playing.
-So far these have all been pretty narrative driven modules though so I'll also recommend
Tortured Hearts II as one of the more massive and open-world modules out there. This one is huge, just genuinely massive. Probably on its own packing more content than the OC and an expansion combined, maybe both expansions. Hell this module pretty much
is an expansion for all the content it delivers, it just also happens to be fan made and free. It's just massive, we're talking hundreds of locations, thousands of lines of dialogue, hundreds of unique items made specifically for this module, and even its own unique crafting systems and the like put together by a small team of developers working together.