Talk to your city rep in your faction's tent. You must have completed the entire Valiant quest line, including the Valiant's Challenge, to take part in this quest.
As before, your rep will send you out to Corp'rethar, the Horror Gate, to assist the crusaders who've set up camp there. You should be familiar with the area by now.
Mount up on your warhorse and get ready, because this challenge is a little different than what you're used to.
Head south from the camp. Use your Shield-Breaker to kill ten Boneguard Scouts and move past them to come face-to-face with the tougher groups.
Unlike previous sorties down here, you'll receive no credit for killing the Boneguard Footmen. Simply run them through.
Your next task is to defeat five mounted Boneguard Lieutenants. As before, these guys start off with one stack of Defend, so pull them with a Shield-Breaker and cut them down with Thrust to defeat them. Don't pull Boneguard Commanders with them or you may have quite a bit of trouble.
Now for the Commander. He starts with three stacks of Defend and a much smarter AI than you're used to with the mounted Valiants you've fought in the past, and definitely smarter than the Lieutenants. Use everything you've learned in your training to take them down -- Shield-breaker and Charge for dropping his defend stacks, strategic running away for Charges and Thrusts, etc. It's a tough fight, but at this point your horse can handle much more punishment.
With those objectives completed, head back to your faction's tent and receive your reward.
Reward: Champion's Seal x1, your choice of Champion's Writ or Champion's Purse, and 13g 23s