Continue your journey with Prima's Dark Souls 2 Guide and Walkthrough!

Reward: Royal Rat Authority Soul, Rat Tail 

When you enter the mist, the boss battle does not begin immediately. Four rats wait on the far left side of the room, with the Royal Rat Authority on the platform just above them. The rats inflict toxic, so be ready to combat that if necessary. 

The best strategy against the rats is to use a weapon with a wide attack range that can hit them all if they're grouped together. It's very important to kill the four rats as quickly as possible so you can focus on the Royal Rat Authority, but you only have a short time to do this before the Authority strikes. 

Head toward the rats or use a ranged attack to engage these enemies. Back up until at least three of them are grouped together close enough to hit them all with a single attack. If you're decently geared, a single attack should take down the rats. Even if you need two attacks, as long as you hit more than one rat at a time, you shouldn't have trouble killing the rats before the Authority begins attacking.

Once the rats are down, you can properly deal with the Royal Rat Authority. The best course of action is to run between its legs, stand directly under its body and attack the hind legs aggressively. If you linger in front of the Authority, it uses two horizontal swiping attacks with its front paws. If blocked, the attacks consume a large amount of stamina when combined. However, there's enough time after the second attack to recover your stamina before the Authority attacks again. 

If you get too far away, the Authority charges, diving headfirst in your direction. If you block this attack, it consumes all of your stamina. However, like the paw attacks, you have time to replenish stamina before the next attack. 

All of the Authority's attacks are difficult to evade, which means your best bet is to block the dive attack and immediately move into place directly under the boss. Stay there as long as possible attacking the hind legs, then move with the boss if it jumps away. Keep in mind, if it jumps too far away, it may attempt the diving attack again. 

When the boss is close to death, it vomits an acidic liquid that must be avoided. It spreads quickly, which means you need to move away, essentially resetting the fight and allowing the boss an opportunity to use the dive attack again. It's best to finish off the boss as soon as you see the yellow vomit appear. If you can't finish it off, you'll need to move away, wait for it to come to you again, then reposition yourself under its legs for another attack. 

Defeat the boss, then you can either take the bonfire back to the first bonfire you visited in the area (Gyrm's Respite in Doors of Pharros), or if you wish to join the rat covenant, head through the small opening opposite where the boss started the battle. Head into the room on the left and speak with the Rat King.

When you arrive at the first bonfire in the area, head left and out into the open, then up the stairs. Kill the enemy in front of the chest to the left, then open the chest to find a Dragon Charm. Head past the fire and into the next room to find a Lifegem in the corner. Continue up the ramp, through the doorway at the top and down the hall to engage the next enemy. He throws both of his axes as you approach, so be ready to guard before you reach him. 

Continue down the hallway, take out the bug on the ground before it disappears, then head down the hall to the right. Kill the enemy inside, then head back and continue down the corridor and engage the enemy ahead. Be careful not to fall off while you battle.

Head into the doorway and up the stairs. Break the pottery on the left, just beyond the fountain to find a Radiant Lifegem, then go through the doorway to enter Brightstone Cove Tseldora.