Rules of Eternus (Energy Substitution)
Elemental Energy Types: Elemental energy substitution powers summon planar energy or substance that the user wields in place of their normal damage type to deal damage to their targets. Each elemental magic type comes with a corresponding energy weakness from the opposing types.
The 20 Opposing Elemental Energy Types: A person cannot take two opposing energy types as a skill unless it is a class ability. Most creatures are resistant to and as such take reduced damage from an energy type that they cast. This is not always the case however.