Chaos energy

Chaos energy (or chaotic energy) is fel.

The chaos magics of the eredar warlocks have burnt out whole worlds and annihilated countless species over the aeons. The Eredar used their warlock magics to enslave a number of worlds that they had invaded. The indigenous of those races of those worlds were mutated by the Eredar's chaotic powers and turned into demons themselves.

On Azeroth, Setesh presided over chaos. Given the purpose of the Halls of Origination, the titans deemed it necessary to create Setesh, a construct who held sway over the powers of destruction.

In World of Warcraft
Chaos is a multi-school damage type that is a combination of all other damage types. The warlock and demon hunter classes have a number of chaos spells.

In the RPG
Chaos energy is highly dangerous; some believe it actually is equivalent to arcane magic, although it can be harnessed by tinkers. Thought at first to be magical, as it can sometimes mimic the effects of arcane magic, chaos energy is purely mechanical in nature. Chaos energy is generated through the use of devices called chaos generators. A small box contains magnets, several differing kinds of metal, steam energy and a tiny golden core that vibrates at a high frequency when powered. Political maneuverings by wizards in Theramore have caused the ruling body to declare chaos generators unlawful, but the goblins and some gnomes still manage to research and develop new uses for chaos energy. The Horde hasn't outlawed the usage yet, as it is not arcane magic, but they are suspicious of it because of the similarities.

Speculation
Setesh's chaos abilities are related to the Void and shadow rather than fel. The Dungeon Journal describes his abilities as containing void energy, During the encounter, Setesh summons Chaos Portals that spawn void creatures. This contradicts Blizzard's later stance that fel is chaos. Given the titans' vulnerability to fel, it seems unlikely that they would employ it on a planetary scale in the Forge of Origination. Setesh's abilities may merely be "chaos" in name.