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Similar to windows ini files, gas files store data. In fact, gas editing is pretty much all you need to know to do some basic modding, as adding new items and spells!

Siege University

A Lorebook is just what it sounds like - a book. Use these to tell your story. Here's how to create lorebooks, which largely consists of a simple template.
What could a mood be if not a template? There's quite a few settings you can change, everything from making it start raining to making everything around the player pitch black.
While quests can be made mostly from the siegeeditor, chapters, which this tutorial also looks into, is more similar to gas editing.
While you won't need to do any gas-editing to just play sounds and voices, you will need to do it if you are to add any.
To add a new object to Dungeon Siege, you add a template. To modify an object, you modify a template. That said, this is where as good as all modders start, and small changes can give large consequences.

Other Resources

Interested2's Gas File Tutorials
An explaination for the logic and stucture behind all kinds of gas files to the prospective modder.
Kurrem-Hoerkin's nifty Gas Tutorial
You know, when you untank those .dsres, .dsparty and .dssave files, did you notice those files with the extension .gas?
Wolfs Guide To Programming Skills
This document is to help people add skill classes and skills to Dungeon Siege. It's a detailed guide to adding a Fire Skill class, with the example of adding a Meteor skill to the game, under the new Fire Skill class.
Zaph's Chapter Interface Tutorial
This was written after we'd figured out how to get the Chapter interfaces working. There are a couple of ommissions in Siege U 209 about firing the Chapter interface and updating the journal. After I'd done it once, and explained bits and pieces here and there, the question was raised again, so I then went through the whole process again, writing it up step by step, in one place so that others would be able to follow.

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