Reward: Dragonrider Soul
Fighting the Dragonrider boss is similar to the first time you fought him in Heide's Tower. The main difference here is that there are now two of them. One uses a bow and remains in the corner of the room for the most part, while the other is almost identical to the one you fought originally.
The best strategy is to take down the ranged Dragonrider first. However, to do this you need to lure the other one to the far side of the room. When he misses an attack, quickly run over to the ranged fighter and inflict as much damage as possible. If you stay as close to the ranged fighter as possible, its arrows will not hit you.
Keep a close eye on the other Dragonrider, and move away when it approaches. Repeat this process until the ranged Dragonrider is down. At this point, you can focus exclusively on the melee Dragonrider. Simply strafe around him or dodge to avoid its attacks, then counter with two or three strikes depending on the speed of your weapon. It won't take long to finish off the remaining Dragonrider.
Head through the doorway and light the bonfire, then continue down the hallway and to the right when you reach the small circular room. Climb the ladder, then head around and open the gate. The statues inside the next room come to life and attack. Move slowly through the room so you don't engage too many statues at once.
Take out all of the enemies, then head through the door and open the coffers in the next room to find an Old Knight Hammer in the first one, and a Caitha's Chime and Soul Greatsword in the second. Continue through the doorway at the end, then around and up the large stairs. Head to the right at the top of the stairs and down the hallway to engage the two archers at the end. Be careful of the archers on the other side that attack while you battle.
Open the coffer at the end of the hall to obtain a Firestorm, then head to the other side, take down the two archers there and open the chest at the end. Be careful of the chest, as it's rigged with bolts that shoot multiple times upon opening the chest. Block or evade the bolts, then look inside to find 10 Fire Greatarrows.
Head back down the stairs and take out the three enemies at the end of the room, then head through the door at the end. Take down the enemy that attacks in the area beyond, then head down the stairs and through the door. Inside the next room, pick up five Repair Powders and 10 Flame Butterflies, then open the coffer at the end to find a Estus Flask Shard. Pull the lever to open the gate in the corner, then open the door. The stairs ahead lead back to the bonfire.
From the bonfire, head through the doorway to reach the circular room and step on the plate in the middle to reach the upper floor. Go through the doors ahead to find a chained prisoner who requires the Key to the Embedded. You do not have the key yet, but remember this location when you obtain the key from the Demon of Song. Once you have the key, you can unlock the cell to obtain a Ring of the Dead. Open the coffers in the room to find a Strong Magic Shield in one, a Soul Vessel and Fire Seed in the second and the Key to King's Passage in the third. Go back to the lift and pull the lever, then head down to the floor below. Use the key to open the door ahead, then proceed down the hallway into King's Passage.