Queen Azshara

Queen of the Highborne

 
Azshara was queen of the elves many centuries before the  beginning of the First War.  She was beloved by her people and her counselor Xavius convinced her to use the Well of Eternity for the good of her people.  
 
But when she looked into the Well she was seduced by promises from Sargeras.  These ranged from the ability to reshape the world to be a perfect one to being Sargeras' bride.   He convinced her to  open a portal into Azeroth.  Mannoroth and Archimonde came through the portal with the plan to strengthen the portal  till Sargeras could come through.   
 
At the final moment a force including night elves, red dragons and time traveling mages stormed the castle and were able to defeat the highborne and destroy the well of Eternity.     

Transformed

 After the battle in her palace the Well of Eternity she collected several of her faithful followers and made a magic bubble to protect them from  the waters of the Well of Eternity.   This allowed them to breath under water.  Hearing voices that she could win by letting go of the bubble she did.   
 
Instead of drowning however she and her followers were transformed by their malice into Naga.  They gained the ability to breath under water and were wholly transformed from elves into Naga.   She set about the work of building a great city under the Ocean bordering an ocean trench.     
 
Many centuries later she sent forth an expeditionary force of Naga led by Lady Vashj.