This quest can not be undertaken until Act III, Chapter 2 (The Morden Chief) is underway or thereafter. Head out of Kalrathia's east gate onto The Eastern Plain of Tears and follow the path, eventually turning north toward The Ruins of Okaym. Continue a short distance further north into The City of Darthrul. They'll have to battle a few enemies at the gate, but will then walk into the center of the city unopposed. The Morden-Viir Patrol Leader, who found the fountain working again in Kalrathia, then spies them and recognizes them as the culprits and a battle erupts. Defeat him and his minions to receive the Key to the District of the Sword.

Continue north into the city and open the eastern door, with a Morden sword on each side, with the key they just received. You can see the first Morden-Viir City Lieutenant off to the southeast as they enter.

They'll find plenty of Morden soldiers inside, who appear none too happy to see them. Once most or all of them have been taken care of, find the first Morden-Viir City Lieutenant and dispatch him to receive the Key to the District of the Crossbow.

Return to the main street running north - south and proceed south back to the center. Head east, skirting the hole, with burning fires in the center, and open the southern door, with a Morden crossbow on each side, with the key they just received.

They'll again find plenty of Morden soldiers inside, who are none too happy to see them. Once most or all of them have been taken care of, find the second Morden-Viir City Lieutenant and dispatch him to receive the Key to the District of the Lance.

Again return to the main street running north - south and proceed north from to the center, skirting the hole, with burning fires in the center, and open the western door, with a Morden lance on each side, with the key they just received.

They'll find more Morden soldiers inside, guarding the City Lieutenant, who will attack as soon as they enter. Once most or all of them have been taken care of, find the third Morden-Viir City Lieutenant and dispatch him to receive the Key to the District of the Chief.

Your party needs to speak with, then battle and defeat the chief and his minions in order to further the primary quest. They do not need to do so to advance this secondary quest. The Chief can be found at the north end of the main north-south street. Use the just received key at the double doors at the north end of the street.

Head inside and up the stairs, then turn east and cross the courtyard and enter the chief's headquarters. A battle ensues after a brief conversation. Defeat the chief to receive his head, which is to be returned to young Lord Kalrathia back in the town of Kalrathia.

But what about the Morden Riders? There are two elevators within the City of Darthrul, one they now have access to and the other not yet. Head south from the District of the Chief and re-enter the District of the Lance to the west. Head into the enclosed area straight ahead. They'll find an elevator in the back room. Activate the switch so they can ride the elevator down to A Cellar Beneath Darthrul. Head for the west side of the room and open the door. Head down the stairway and engage the last of the Morden-Viir City Lieutenants, who drops the key to the District of the Shield, when dispatched.

Return to the main north-south street and head south to the center intersection, turning east to skirt the hole, with burning fires in the center, and continue east until encountering the door to the south with Morden shields to either side. Use the recently received key to open the door.

They'll of course be met by the requisite number of Morden enemies as soon as they enter. Their destination is an area below ground level due south along the outer wall (my two lead characters are headed straight for it). Once there, some diligent searching will uncover a switch hidden low on the western wall. I completely missed this switch on my first time through the game and had to ask for detailed help to find it. Activate this switch, to ride the elevator down to a room, allowing access to the southern and eastern areas outside of the walls of Darthrul. The only remaining problem to initiating and completing this quest is the enemies they'll encounter between now and then.

Have then head outside and head east toward the southeast corner of Darthrul, where they'll encounter stairs heading up to the south end of the east side. Follow the path to the east, continuing along as it turns to the north. Head north a bit farther and they'll encounter Nordax and Nelliok, who're Morden-Viir Riders, and Bruno the Klask in front of a wall. These three Morden will not attack your party. You may speak with them instead. Speak to Nordax (the one in the middle) and he and Nelliok will tell a sorry tale of Morden incompetence and also be quite admiring of human architectural accomplishments. You'll learn that they're on the run from their own people, whom they expect would kill them if they returned. They ask you to bring them back some specific supplies from Darthrul (i.e. a Morden Tent, and Morden and Klask Rations), so they can make their escape. Agree to do so to initiate this quest and receive the Darthrul Supply Key.

Return south, then west to the stairs and head back down to the lower level. Continue back to the west until they encounter the entrance to the room containing the elevator going up into the city. Enter and activate the switch in the floor to ride the elevator back up. Press the 'Tab' key to display the larger map and you'll see the three storeroom locations, each marked with a slowly pulsing pale white star. They'll encounter the first Supply Vault Door to the immediate northeast, as soon as they've climbed the steps from the upper elevator area. Approach it and left click on it to use the Darthrul Supply Key.

Return north back out onto the main East-West street of Darthrul and head back toward the center of town. Head north from the center, skirting the hole, with burning fires in the center, and re-enter the District of the Lance through the western door with a Morden lance on each side. Proceed southwest a few steps and use the Morden Supply key to open the door to the second Supply Vault.

Head back out onto the main North-South street through the door they entered the area from and turn to the south. Proceed back to the center of town and turn west at the teleporter, continuing under the broken bridge, and out to the end of the street. Re-enter the District of the Crossbow through the southern door with a Morden crossbow on each side. Head out of the initial room through the door to the east. The third supply vault is immediately to their right as they enter the next area. Move around to its east face and use the Morden Supply key to open the door to the third Supply Vault.

Return to the main East-West street and head east back over to the other side of town, until encountering the door to the south with Morden shields to either side, which is now open. Re-enter the District of the Shield and head straight back and down the stairs. Activate the elevator switch to ride the elevator down to the lower level. Head on outside and proceed east until they again reach the stairway to the north. Have them climb the stairs, then continue east, then north until they again encounter the three Morden deserters. Speak with Nordax and he'll thank them for bringing the requested supplies and mention that they don't have anything with which to repay them for their trouble, but there is some treasure to be found on the other side of the wall behind them and orders Bruno to knock it down, which he does. This completes the quest.

Unfortunately the area behind the wall is also crawling with enemies. They can go ahead back there and clear out the area then loot the Big Chest within the decrepit building if you wish.