Online:Slithering Brood

Quick Walkthrough
# Talk to Regent Cassipia in the abandoned farm outside Dragonstar.
# Defeat Lakorrah the Matron‎ in Fearfangs Cavern.
# Defeat Takara the Eggbearer‎ in Sunken Lair.
# Speak to Cassipia outside Dragonstar again.
# Find the Spawning Pools in Serpent's Nest.
# Defeat the Progenitor Mantikora and free the prisoners.
# Talk to Regent Cassipia.

Detailed Walkthrough
Once you have completed the quest The Missing Guardian, Thaenaneth will be waiting for you outside the Star-Gazers' Observatory. She informs you that her patron, Regent Cassipia, has plans for the Scaled Court which are different to the plans of the Celestial Serpent. She directs you to an abandoned farm outside Dragonstar if you are interested in finding out more.

This quest can also be started by talking to Cassipia outside Dragonstar directly, or after completing The Serpent's Fang and being directed there by Little Leaf through Holding Court.

Regent Cassipia wants to sabotage the Serpent's operations and make a better world. Although she was a member of the Scaled Court, she and Little Leaf are working to oppose the Serpent. She accompanies you on your mission.

The Scorpion Matron
The first on Cassipia's hit list is Lakorrah the Matron‎, a giant scorpion matron that the Scaled Court are using for breeding purposes. She is the final boss of Fearfangs Cavern to the west. After killing her, your group will need to destroy the nearby egg mounds before continuing.

Lightning Dragons
After defeating Lakorrah, Cassipia will tell you to go to Sunken Lair to the south and take out the Scaled Court's wamasu specimen, Takara the Eggbearer‎. Sunken Lair is a small cave inhabited by wamasu and Court members, although Takara has powerful shock attacks that can take out your entire group if you don't dodge. While in the cave, you will also need to destroy another egg mound.

Once this is done, head back to the Dragonstar Wayshrine and reunite with the Regent. She will tell you that the final step in her plan involves slaying the original Progenitor Mantikora, who can be found tending the Spawning Pools in the depths of Serpent's Nest to the northeast.

The Spawning Pools
Inside Serpent's Nest, you face Laurieae, Taurieae and Aurieae, three lamias. However, you can skip this fight by sneaking behind the rocks to the left. 

Enter the Spawning Pools Chamber and kill the Progenitor Mantikora, Hruzizolk. Hruzizolk uses a spear, and you'll also have to manage the members of the Scaled Court and the mantikora that spawn in the four corner pools. These mantikoras get bigger over time, so it pays off to defeat them quickly. You will not be able to release the prisoners until all the mantikora have been defeated. 

Free the four prisoners on the east and west sides of the room, then follow Cassipia as she confronts Regent Boward.
:Regent Cassipia: "Present yourself, Regent Boward. the mantikora project is at an end."
:Regent Boward: "Regent Cassipia? What is the meaning of this?"
:Regent Cassipia': "Watch your tone, regent! Your abomination is dead. Will you abandon the Serpent and join me?"
:Regent Boward: "Abandon the Serpent? I would never do that!"
:Regent Cassipia: "Very well, Boward. Join your abomination in Oblivion!"
Cassipia disintegrates Boward on the spot, and asks you to meet her outside.

She thanks your for your help, and rewards you with an  . If you have not yet completed The Serpent's Fang, she asks you to seek out Little Leaf near the northeast passage, starting the quest A Leaf in the Wind.

Notes
Don't use the portal at the end of the fight with Hruzizolk if you haven't yet obtained the skyshard in Serpent's Nest, as this will force you to fight your way through the entire dungeon again.
 

Quest Stages