Continuum Blade

The Continuum Blade is a very good caster sword with high spell damage, while easily obtainable with only a few Caverns of Time reputation runs. It is one of the best pre-Karazhan tanking sword for protection paladins because of the spell damage and high stamina.

Source
You can buy this sword from the Keepers of Time quartermaster when you are.

Trivia

 * Continuum Blade is a Rare Weapon card in the World of Warcraft: Trading Card Game's Drums of War set.