You have found one of the remaining members of the Company of Heroes and ask him what he knows about Titan.
This quest is available if you have completed the quest Wrath of the Titan.
Starting the Quest
Talk to Trachtoum in Lower La Noscea to begin the quest. There will be a cutscene.
Destroy the Rats' Nest
Exit the building and go south to the quest destination. You will find a purple glowing Rats' Nest. Check on it, then kill the four Wharf Rats that attack. After that, kill the Rat King that appears.
Speak with Trachtoum
Go north and go back into the building and talk to Trachtoum to complete the quest.
Next Main Scenario Quest
After you complete the quest, talk to Trachtoum to accept the next quest: Hungry Hungry Goobbues.