The game rules used in Obormot's Reign of Winter are based on the Worlds of Adventure rule set, with the E6 variant applied. (The Worlds of Adventure rules are loosely similar to the various "D20 System" rulesets, such as D&D 3.5 and Pathfinder.)
Links
Character Creation
All about making a new character.
Character Advancement
How "leveling up" works (up to 6th level).
The E6 System
What happens after getting to 6th level.
Classes
The special rules for the various character classes, and the epic options for each class.
Worlds of Adventure is a work in progress, but Obormot's Reign of Winter doesn't use the whole W.o.A. rule set — just some of it. This site, for the most part, lists only those rules and game mechanics which are modified from Worlds of Adventure; most of the rules used in this campaign can be found on the Worlds of Adventure site. You should assume that everything in Obormot's Reign of Winter works like it does in Worlds of Adventure, unless stated otherwise.
The key differences in rules between Obormot's Reign of Winter and Worlds of Adventure are as follows.
The E6 System
Characters in Obormot's Reign of Winter reach the 6th character level, then stop gaining levels. They don't stop improving or gaining new abilities; their level just stops increasing (as do the various numbers linked to character level — base attack bonus, saving throws, and so forth). See The E6 System for the full details, and also see Character Advancement for information about getting to 6th level.
Classes
Characters in Obormot's Reign of Winter can select any of fifteen classes. Each class is modified somewhat from the way it works in the Worlds of Adventures campaign, both in order to work with the E6 system, and to fit the needs of the campaign. The Classes page lists all of the available character classes, and each individual class page has detailed information.
Feats
Obormot's Reign of Winter has a smaller list of available feats than the Worlds of Adventure campaign, by necessity. The Feats Index lists all of the feats available to player characters. (Note that certain classes gain access to additional feats, as class-specific epic advancement options; see each class's page for details.)
Magic
Obormot's Reign of Winter contains certain magical elements — spells, phenomena, etc. — that are specific to this campaign. One of these is the opposition between fire and ice, which is described on the Fire vs. Ice page.
Other Stuff
Worlds of Adventure is a larger ruleset than is needed for Obormot's Reign of Winter, because it's designed for a campaign of much broader scope, and greater complexity. Accordingly, the following parts of the Worlds of Adventure rules are not used in Obormot's Reign of Winter:
- Archetypes: Characters in Obormot's Reign of Winter have access to the standard versions of each class only, and don't have the option of choosing an archetype.
- Crafting: The general magic item creation system in the Worlds of Adventure campaign is not available in Obormot's Reign of Winter (player characters will not have the time or opportunity to engage in extended crafting, or to pursue lengthy side-quests for magical components), although player characters can scribe scrolls (with the Arcane Script skill) and make potions and other alchemical items (with the Alchemy skill).
- Research: Characters in Obormot's Reign of Winter generally don't have the time or resources to conduct research (magical, alchemical, martial, or otherwise).
- Leadership: Characters in Obormot's Reign of Winter don't attract followers (although certain NPCs may, on occasion, choose to travel with the party for a time) or found organizations.
- Reaction, Morale, Reputation: These complex social mechanics are unnecessary in Obormot's Reign of Winter, and are not used.
- Psionics: Obormot's Reign of Winter does not include psionic characters or powers.
- Cross-Classing: The E6 system used in Obormot's Reign of Winter is incompatible with the Worlds of Adventure cross-classing mechanics. A character in Obormot's Reign of Winter may "multiclass" by taking the Secondary Class epic advancement option. (Note, however, that the classification of class groups still matters, for the purposes of using magic items, ability to use certain combat reactions, and some other aspects of the rules.)
- Beyond 20th Level: The epic mechanics used in the Worlds of Adventure campaign are not compatible with Obormot's Reign of Winter.
