Note - the names and story that follow may be very familiar to those who are familiar with Celtic legends. 
 

History:

Nuada Silverhand is the King (a gender neutral title) of the Atlantean city of Thierna Na Oge.  She was chosen to be king because she was the most powerful mage.    She survived some ordeal but lost her right arm.  It was replaced with a silver mystic prosthetic by the physician Dianecht.  Because of the loss of her arm, the Conclave (a ruling counsel of 12 mages) felt her unfit to continue being king. Her throne was usurped by her sister Bres. 
 
In a plot by Ocean Master to further his mystic abilities, he sought an artifact (the Lia Fail) from Thierna Na Oge.  The artifact was stolen, Nuada and Aquaman were blamed.  After escaping imprisonment Nuada worked with Aquaman in defeating Ocean Master and recovering the artifact.  This occurred in the Aquaman mini-series written by Neal Pozner.  It may no longer be in continuity.  
 
Nuada reappeared later (in an in continuity issue), retaking the throne from Bres.  Bres threw in her lot with some aliens in the ongoing Aquaman story by Peter David.  Bres was slain.  Nuada assists Aquaman in fighting off the aliens in the continuing story arc, and regains her throne.
 

Powers and Abilities:

Along with the standard Atlantean abilities, Nuada can cast elemental magic.  She is the mightiest mage in Thierna Na Oge, an Atlantean city where mages are very common.  She also has access to the Lia Fail artifact, an item of vast mystic power.