After the Persian Occupation of the Inner City ends, Scarlett can visit the Tavern again. She will find the Guard Captain of the Inner City has left the Tavern and return to the Garrison. At the table where the guard captain previously set, she will find Callisto. If she speaks with him, he will implore her to help him with his quest.
A young man, Callisto, has a noblewoman for a lover. She is one of two rival siblings to be heirs to a island estate. (We can infer the woman's name is Sara.) Her brother has hired Assassins to kidnap her and hold her for 1000 ducats ransom. Callisto isn't wealthy and doesn't have that much money, but he implores Scarlett to help him.
After getting the quest from Callisto in the Tavern, Scarlett will need to visit the Guard Captain of the Inner City at the Garrison to ask him where to look for Assassins. He'll tell her some suspicious-looking men were seen entering the Catacombs recently.
In the Catacombs, Scarlett should head to the chamber illuminated by the two magical floating balls of light. (There are two possible routes to that chamber; turning left and following the platform until a ramp to the left enters the water where Scarlett can swim to the corridor entrance is the shortest, but it doesn't really matter which way she goes. (Stay alert for Grippers wandering about.)
After paying the ransom and watching Sara leave, Scarlett has a choice of leaving the Assassins or attacking them. While it is not philosophically obvious which course is honorable and which villainous, killing them has the advantage that Scarlett can recover her ransom money (and a little loot as well.)
While still in the Inner City Catacombs, Sara will give Scarlett her first part of the reward, a Ring of Life (a vey useful magical item which greatly increases the rate at which Scarlett recovers Health.)
Rescuing Sara will open her Western Villa to travel there by Gondola (the only way to reach the Villa.) Therefore, it is a prerequisite to dig up King Draco's Treasure buried there and earn the Treasure Hunter Achievement (Trophy).