After completing some quests around Cassardis and the Encampment (see previous chapters of the walkthrough), it's time to continue with the main story quest, Off With Its Head. You first need to regroup with Ser Mercedes at the Mountain Waycastle. It is best to travel during the daylight, so if it is currently night time, rest at an inn until the morning

From the encampment, go northwest on the Manamia Trail. There are some armed bandits camped in an Abandoned Storehouse along the way, you can kill them or just dash through, ignoring them (unless they attack you). You can keep following the road, or cut through the mountains to the west. There is a chance you will run into more pawns along the way, which you can enlist (replacing your own pawns). The road splits several times into different ways, so check your map often to get beatings. You should eventually see a big fort/castle on the distance (to your right), and reach a gate near where a cart is parked, with the Hydra head on it. This is the Mountain Waycastle.

Speak with Ser Mercedes, standing near the caravan. She should have a red exclamation mark above her head, signifying interaction needed for quest progression:

Mercedes: I trust you rested well, given the hour. The Arisen arose late this day, eh? We make for the Capital, men. Like as not, the cat will draw beasts along the way. Stay vigilant! But fear not. We walk with the Arisen, slayer of hydras!

The gate rises. Once it is open, your goal updates, to Escort to the Capital. There is a riftstone just before the gate, which you can use to get new pawns if you want. You want to have at least one ranged class, in your party, as you will be attacked by monsters such as a Harpy. The cart moves slowly, and you should stick with it, despite any urges to run forward. Allow no monsters to approach the cart. You cross a stone bridge at first, and are ambushed by goblins as soon as you reach he other end.  Defeat them, and stay at the cart's side. Watch both ground and sky for threats, as you will be attacked by Harpies as well. Also, keep watch for falling rocks on the path ahead, so as not to become trapped under a rock slide. With so many twists of the road, it's hard to see whats coming up.

The path diverges, and the Ox will turn right. There will be more Harpy attacks. If a Harpy gets ahold of you and tries to lift you up, follow the on-screen prompts to break free. As you exit the canyon, wolves join the fight. Wolves are vulnerable to fire, so use fire and flames attacks against them whenever possible.

Eventually you reach a locked gate. Pull the lever to lift the closed gate. A goblin pack storms the caravan. Defeat them. They will be attacking in waves. Then, make for the Capital. At this point you are past more than half between the encampment and the capital. The canyon floor is an apt spot to find yourselves surrounded, so be careful. You need to stay vigilant until you are in Gran Soren's gates. Soon you will see the massive gates of Gran Soren in the distance. There will be some final waves of goblins attacking, and then some bandits. Bandits attack in quick succession, but are left exposed after a grand strike, so counter after they strike.

You soon reach Gran Soren, the capital of Gransys. Approach the guards to trigger a cutscene:

Mercedes: We've arrived. Well done, men. A Hydra's head is a fine prize, his grace will be pleased with the Corps's work. Walk proud through these streets, men. We are heroes to the people of Gran Soren.

You enter Grand Soren. There is another cinematic. Mercedes says she will make for the castle directly to offer her report to the Duke. She suggest you hang out in the capital for a while.

This completes the quest, Off With Its Head.