From the Poison Pool bonfire, head out of the cave and turn right. At the end of the path is Chloanne, a merchant who sells Titanite Shards, Bonfire Ascetic, Soul Appease and Dead Again. It’s not worth buying Titanite Shards from her as many of the enemies in this area drop them.
Follow the path found and you’ll come to a large expanse with a poison gas pool in the middle. Turn left and a skeleton will spawn, quickly kill it before it forms. The giant in the centre won’t attack you until you run by or attack it. It swipes with its arms and throws dark orbs which do huge damage, but can be easily dodged. You can kill the giant relatively easily, but must do so quickly, as you’ll also be dealing with the poison gas.
The best strategy is to run around and collect all the items on your first attempt, then come back and head into the cave on the opposite side of the entrance and speak to Gavlan the merchant. The items in this first area are:
- Fragrant branch of Yore
- Torch x 2
- Fire seed
- Divine blessing
- Fading soul
- Smooth silky stone
- Titanite chunk
- Poison throwing knife
On the second run, head into the cave and Gavlan is up the path to the left (out of the poison gas). Kill the skeleton before speaking to him. Take all of the items you wish to sell before speaking to him, as he’ll disappear after the transaction. There’s a chest containing a Raw Stone next to Gavlan, too.
Head back out and exit this area (up the other hill along the left hand wall from the route into the poison pit from the entrance), you follow a winding path leading to two more giants and some zombies up a couple of ladders. Before taking on the giants, head up the first ladder to the right and grab the Hexing Urn. Drop back down and kill all zombies and the two giants before taking the second ladder. Pull the lever to open the gate and the second bonfire is just to the right.
After resting at the bonfire, head out and turn right, you’ll come to a second ‘hub’ area filled with holes in the floor. Head down the ladder immediately left, sprint through the poison gas and up the ladder at the end. Don’t worry if you get poisoned, you’re just here to collect the Poison Stone and Rotten Pine Resin from the chest. You can run back to the bonfire and start again instead of using an Estus Flask.
Back in the hub area, continue along the left hand wall and through the gap, head over the rubble (don’t turn right just yet, we’ll come to that in a moment). At the end of the path, a giant will break through the wall; use him to break the second wall at the back before killing him to grab a ton of goodies; including a Soul of a Lost Undead, Old Knight Pike, Old Knight Greatshield, Torch, Radiant Lifegem and a Smooth and Silky Stone.
Head back over the rubble and turn left. there’s a huge ambush waiting for you in the form of four sickle-wielding foes, but it’s worth entering here as there’s a ton of items behind the wooden doorways. You can actually survive without engaging any of the enemies, just keep sprinting around the large area, hitting the doors and doing laps to avoid the fight. You can kill the zombies in the area, but avoid the sickle guys. If you get cornered in one of the rooms, you’re pretty much dead, but keep coming back here until you’ve looted all of the items. One of the doorways leads back to the main hub, so rest at the bonfire once all items are collected to reset the enemies and continue exploring.
After you’re done here, head through the wooden doorway to the right of the hub. There’s a large poison gas pool with two giants in the middle of it. Stay clear of this and follow the ledge to the right. You’ll come across a new enemy hear: a huge hammer-wielding foe that does massive damage. He’s weak to fire, so backstep to avoid attacks and hit back with single strikes only.
Follow the path to the left, there’s a sickle enemy and an altar to join the Altar of Sunlight covenant. Backtrack to the path and enter the building at the far end of the ledge. There’s two more hammer guys in here, take them out one at a time. The first hammer guy should be pretty easy to coax outside. Head up the ladder on the far left and backstab the final hammer enemy, being mindful of the poison pots that surround you. The doorways here lead to Earthen Peak.
Dark Souls 2 Guide - Harvest Valley Walkthrough