Speak with Deathstalker Visceri at the Argent Tournament Grounds to become a valiant of the Undercity.
You've proven yourself worthy of competing under the banner of the Undercity, <name>. Now, you should speak with Deathstalker Visceri, who leads the Undercity delegation. As grand champion, he will enter you into the tournament on behalf of the city and oversee your training and testing as valiant.
It has been an honor working with you, <name>, and I wish you luck in the competition ahead.