"Finding Mayhew" is the final quest sequence which will allow the player to enter Mayhew through the sewers.
Summary and Walkthrough
After outright destroying the goblin fortress or after completing the siege quests of Siege of the Ratman Fortress or Siege of the Goblin Fortification the player can travel to the Mayhew Tunnel. A random encounter will allow the player to start looking for an exit. Next the hero needs to storm a ratmen encampment (containing around 20 goblins of various types) and free a group of merchants from captivity. The merchants flee in the chaos, so you'll have to explore to find them again. When you do, you can escort them back to Mayhew and complete the quest, though if you keep asking for a reward they will decline and you'll have to find them again.
Note: If you have already completed the Stop the Ratman Raids quest the exit to Mayhew will be blocked and the poor merchants are left to their grim fate. This also completes the quests.