This quest begins immediately after Clues and Riddles. Now that you’ve spoken with Halewyn, you’ll need to meet Modron at the Rollendritch stones, located north of the druid’s house.
Follow Modron to Thieves Warren
When you speak with Modron she will agree to show you the way to Thieves Warren. Follow her along the road until she mentions that it’s time to part ways for a while. You can then follow the road northwest until you reach Thieves Warren. If you approach the camp from the east you’ll find a hole in the defences, which you can use to enter unseen. Once inside, you’ll hear Tewdwr’s voice, confirming that he lives. You’ll need to follow it to where it’s coming from, in a building at the northwestern side of the camp.
You can enter the building after killing only those guards which are necessary to reach it by stealth, or you can fight them all in open combat. The choice is yours, but note that if you attract the attention of all the guards, you will have to contend with a lot of incoming ranged fire from slingshots. You’ll find plenty of fire pots throughout the camp, which you can lure enemies to and then shoot to cause an explosion. However you do it, once you’ve got an open path to the building where Tewdwr is (use Odin’s Sight if you need to locate it), enter it to begin a cutscene.
What to say to Gwilim and Modron
During the scene you’ll have a few dialogue options with Gwilim. It doesn’t matter which you choose, as the outcome will be the same. After the scene, pick up Tewdwr and take him out of the camp to the south, where Modron is waiting. It doesn’t matter what you tell Modron with regard to what you intend to do with Cynon, so choose as you wish here. When the scene ends, the quest will complete.