Return to Huntsman's Copse and fight the Skeleton Lords.
You need to travel back via a bonfire to the Undead Lockway. When there, go out of the room and across the stone bridge to the right. Follow the path defeating the enemies as you go and take the first fork on the left. There are ten Poison Throwing Knives here.
Drop down on to the path and go over to the waterfall - head down the path to the right of the blocked doorway and circle all the way around until you reach a cave.
Inside the cave you'll have to fight a number of skeletons. Take the three Flame Butterflies, pull the lever which will connect the two parts of the cave. Go back to the waterfall and up the stairs underneath it to enter the mist.
Boss Battle: Skeleton Lords
Reward: Skeleton Lord's Soul
Here you have to fight three bosses and a horde of other skeletons. They all go down fairly quickly but it's easy to become overwhelmed. As soon as the fight begins take on the first boss and kill it. If you're quick you can kill one before the other two set to work on you. When you fight the other two be sure to keep an eye on them both to evade any ranged attacks.
Once those three skeletons are dealt with start on the rest of the horde, focusing on the ones with wheels first. If they rush at you dodge out of their way. Use the arena as much a possible to keep moving and chipping away at the enemies until they are all down.
When they are all dead go through the doorway on the right of the entrance and down the corridor. Pull the lever at the end to lower a bridge. Across the bridge is a corridor and ladder. Follow it and you'll arrive at Harvest Valley.