Continue your journey, find out how to use the Pharros' Lockstones and go to battle against the Royal Rat Authroity.
The first thing to note here is that the enemies in this location are tough to stun. It's better to attack once, then block, then attack again until they go down. Kill the enemy directly ahead of you, then take down the next enemy. Grab the Prism Stones but do not go near the chest - it will come alive and kill you.
The next bonfire is on the right through the opening. Once that's lit go into the monster mouth doorway and kill the dog and the enemy before the invader attacks you. Be careful here as he swings slowly but multiple times. Be certain he's about to pause before you attack.
Bowman Guthry uses two crossbows but it's easy to anticipate his attack as he raises the weapon he's about to fire. He's good at evading attacks though and he uses a Estus Flask to heal himself so don't stray too far from him. Once he's dead you get the Royal Swordsman Armour.
In the far right corner of the room are two Amber Herbs. Now go through the doorway in the far left to get the Gyrm Axe and Torch from the balcony above.
Go back down and across the water, through the doorway on the far right and kill the two rats. Up the ladder on the far side of the room is a Twisted Barricade and Soul of a Proud Knight.
There are two enemies to fight here and a trapped wall with spikes to avoid. The enemy at the end in the alcove will trigger it. As soon as the spikes recede dash over to him and chop him down.
The chest here is also a trap - poison this time - so have something on hand to deal with that and help yourself to the Titanite Chunk and Petrified Dragon Bone.
Follow the path to the far side of the stairs and take the Pharros' Lockstone and Large Soul of a Nameless Soldier. Up the stairs is the room where you started (you're on the upper level now), so go left and enter the doorway to find a bonfire and mist.
Once you've lit the fire go back out of the room and down the path to fight two enemies, then over two bridges for a Large Soul of a Brave Warrior. Now head back to the mist and get ready for a fight.
Boss Battle: Royal Rat Authority
Reward: Royal Rat Authority Soul, Rat Tail
Entering the mist, lay into the four rats immediately using a weapon with a wide arc so you catch them all when they get in close. The Royal Rat Authority is just above them. Take them out quickly and you'll have them dead before you have to fight the boss.
The best tactic with the Royal Rat Authority is to run between its legs and attack its back legs while standing underneath it. Stand in front of it and it uses to swiping attacks with its paws - blocking them sucks up a lot of your stamina. If you move too far away from it, it will charge headfirst into you, again taking most of your stamina if you block.
The Royal Rat is hard to evade so blocking is the main tactic here despite it eating up so much stamina. Hit those hind legs and follow it when it retreats. Once it's close to death it begins to puke out acid with a wide radius. Stay clear and attack again when you get the chance, but at this point you should be concentrating on killing the Royal Rat quickly.
Once its dead you can choose to join the rat covenant. Find the Rat King through the small opening where the fight started.
Now head back to the first bonfire in the area and go left and up the stairs. Kill the enemy guarding the chest for a Dragon Charm. In the next room is a Lifegem and up the ramp at the top you'll find a hall. The enemy here throws axes so be prepared.
Go down the hallway and kill the bug before it flees, go right and kill the enemy before heading back down the corridor and fighting another enemy but be careful not to fall.
Go up the stairs and smash the pots for a Radiant Lifegem. Now go through the doorway to enter Brightstone Cove Tseldora.