Travel to the Scalding Pools located near the top of the Hand of Gul'dan and use the Forged Illidari-Bane Blade to quench it in the lava pools. Once you have the Quenched Illidari-Bane Blade, return to David Wayne at Wayne's Refuge.
You can almost feel the magic beginning to coalesce inside the steel, but one thing remains to be done.
You must take the forged blade to the Scalding Pools near the top of the Hand of Gul'dan in Shadowmoon Valley and plunge it into the lava pools to quench the sword. When you have quenched the sword, return.