You get the mission "Slaying the Beast" after doing "Judge and Executioner"
Open your mission log:
Slaying the Beast
Overseer Tremel has handed down your third trial - you must slay a great beast in the tomb of Marka Ragnos. Journey to the tomb of Marka Ragnos and discover the antechamber containing the ancient Sith carving. Then, meditate on the dark side to summon the beast. Speak to the shuttle droid outside of the Academy and take a taxi to the Lower Wilds.
Your first task is to ride taxi to Lower Wilds. The taxi is just outside Sith Academy, down the ramp. Talk to the Sith Academy Skyhopper and choose the Lower Wilds as your destination. From there, take the road southeast and then southwest, and reach the Wilds Laboratory. From there turn to the north. Enter the Tomb of Marka Ragnos Reliquary. In there, kill 5 Acolyte Deserters to complete the bonus mission "Destroying the Deserters". Work your way through the tunnels, heading towards the NE. Meditate at the Ancient Sith Carving to summon the beast. The beast will appear. Slay it. Then return to Overseer Tremel at the Sith Academy. You should use Quick Travel, if available.
Dolgis is waiting for you in the hallway. He wants to kill you when there are no witnesses. Defeat Dolgis, then speak to him. He will ask you to spare him. Your options are:
You earned this. [Kill him] (DARK)
Don't cross me again. (LIGHT)
Then buy your life. (NEUTRAL)
If you choose to let him go, you tell him that next time he won't be so lucky, and he promises you won't see him again.
Next, proceed to talk to Overseer Tremel in his office. He tells you he may have made a slight miscalculation. The beast of Marka Ragnos was a great source of dark energy here no Korriban. When it was slain, there was a tremor in the Force. Darth Baras felt that tremor, and has become aware of you. He demands an audience. Baras is a serious man but a master of deception. Everything he does and says is calculated. He will attempt to trip you up, test your nature, get to the heart of who you are. Always take him seriously. You're the best chance of stopping Vemrin. If you fail, Tremel doubts there will be another strong enough. Meet Darth Baras in his chambers. And hurry, he won't take kindly to waiting.
This completes the mission "Slaying the Beast" and starts the mission "A New Master".