Continue your journey with Prima's Dark Souls 2 Guide and Walkthrough!
Head into the mouth-like entrance, take out the two zombies on the way in, then the four rodents of unusual size in the room ahead. The rodents come at you from both sides, so be careful when fighting them. Depending on your level, they will likely go down in a single hit. Head around to the far side near the open doorway to find a Large Soul of a Nameless Soldier, then head through the doorway and up the stairs.
Four more rodents attack from the right, but make note of the face on the floor near the top of the stairs, as you'll need to access it later. Take down the rats, then head to the left and kill the zombie at the end. Pick up the item to obtain three Small Smooth & Silky Stones. Use a Pharros' Lockstone in the previously mentioned face if you have one available. This is the only face that you should interact with, as the others do not benefit you; ignore all of them.
If you used a Pharros Lockstone, head across the bridge and into the room just beyond. Go to the right side of the room and kill the rats that spawn, then pick up the item to obtain two poison moss. Continue up the stairs to find a Whisper of Despair and a Torch. You cannot continue forward because the way is blocked, so head back across the bridge and to the far left end of the hall. Climb up the ladder on the right to reach the next floor.
Rhoy the Explorer invades when you reach the top of the stairs, in addition to two more rodents (one attacking from either side). Take down the rodents while avoiding Rhoy, then focus on the invader. Continue through the broken wall to the left to find three Homeward Bones, then head down to the other end of the hall to find two more rodents and a bonfire.
Light the bonfire, then it's time to prepare to head into the mist down the corridor to the right. Inside the mist you will face a plethora of rodents, followed by the Royal Rat Vanguard.