Elemental mastery refers to the powers of the half-brothers. Lysaer s'Ilessid can control elemental Light, and Arithon s'Ffalenn can control elemental Shadow. These powers were bequeathed upon them as the result of a bride's dowry, when Talera s'Ahelas (of the Rauven mages) was wed to the King of Amroth. It has been stated that the Fellowship are quite capable of these powers themselves, but that they would never do so, because what is possible does not always coincide with what is wise.
The author has provided a clarification about what elemental mastery encompasses:
|On Talera's bridegift - you have mistakenly applied the term "elemental powers" to apply to the four elements, earth, air, fire and water. Light and Shadow do not fit into that catagory....nor should they. Elemental power, as Asandir would define it, means that the powers the two princes possess draw directly OFF OF the elements, and in short, influence all of them, without boundaries.
So the power of Light and Shadow do not correlate to earth, air, fire and water, but use the forces of all elements. What drives the powers of the elements - that would be source for the princes' abilities.
A more detailed description of how these powers might work can be found in Peril's Gate:
|A willed state of mind punched a vortex within Lysaer's auric field. Light flowed from the aperture framed by his hand, two threads of harmonic resonance pulled from the grand chord and bidden to manifest as white fire.|
Arithon's gift of Shadow can be engaged reflexively and sustained with minimal effort for extended periods of time.