Return to Silvermoon City and report back to Lor'themar Theron.
Your people suffered a great betrayal by the Alliance. You've succeeded in fending off Darnassian attacks as well as spies from Ironforge. Now you've defeated a powerful Scourge leader at the footsteps of your home. Your worthiness is no longer in question.
It is now apparent that you need us and we need you. Return to Lor'themar. Tell him I received his message loud and clear.
Welcome to the Horde, elf.
Your work has helped advance our race's cause a tremendous deal. Not only does Thrall believe we're capable of handling ourselves in the face of the enemy, he also believes he can use our link with Outland as a means to reach his people.
Not exactly the truth, but not a lie either.
Brace yourself for great changes, <name>. We are now officially part of the Horde.