Speak with the High Priest of Icthlarin in the temple in Sophanem. After learning that the plagues on the city are not contagious, he seeks to lift the quarantine Menaphos has placed on them. You will see a short cutscene where the temple's priests attempt to enter Menaphos unsuccessfully.
When asked what ideas you have to get into Menaphos, respond "Is there any way into Menaphos from below?" You learn that followers of Scabaras, a lesser deity, were great subterranean builders, and that a statue exists in the Temple of the Lesser Gods in the north-east part of the city. Head to the temple (just east of the Pyramid Plunder minigame) and speak with Jex.
Jex tells you that the wrecked temple is the result of an attack by strange scarab creatures, leaving the temple and bank in shambles, and that a dungeon exists in the cellar of the temple. Climb down the trapdoor inside the temple, and use the trapdoor to enter the dungeon, after several warnings from bank guards. If you did not bring a light source, you can buy a torch and tinderbox from either of the bank guards for 250gp (200 for the torch, 50 for the tinderbox). Even if you bring a Bullseye lantern, bring a tinderbox!
For travelling through the Scarab Maze, refer to the following map.
This is a very dangerous dungeon, with level 93 scarab mages, level 98 locust riders, and traps capable of hitting 150 damage or more. Look carefully for scarabs on the walls denoting where trap doors are hidden, and for moving circles on the floor where swarms of scarabs are hiding. If you fall down a trap door, your light will be magically put out, even a bullseye lantern!
Playing the Messenger
When you reach the ladder at the south-east corner of the maze, follow the winding cave path until you find Kaleef's body. Read the bloody parchment and continue into the large room and speak with Maisa. After telling her that Kaleef is dead, she reveals that Osman was her contact to get out of Menaphos. To prove she can trust you, you must answer the following questions:
"Where was the prince being kept?" - Draynor Village
"Who helped you find the prince?" - Leela
After proving yourself trustworthy, travel to Al-Karid and speak with Osman, who is in front of the palace. He will tell you that he is not on good terms with the Menaphite kingdoms, and needs a good reason to travel there. Respond with "It would drive a wedge between the Menaphite kingdoms." He agrees to meet you outside Sophanem.
Meet him outside Sophanem and he tells you he couldn't find a way in to the city. Tell him "I know of a secret entrance to the north." You view a cutscene of Ozman sneaking into the city and into the temple maze, and decide to go look for him after enough time passes.
Return to Maisa, who is finishing up a conversation with Osman. The cave begins to shake and Osman tells Maisa to run. He tries to flee himself, but three Scarab Swarms appear and combine to form a level 191 Giant Scarab, which kills Osman.
Big Bug Bashing
The giant scarab is a particularly tricky foe. It is particularly resistant to Melee attacks, and summons a scarab mage and locust rider. From a distance it will use ranged attacks, but up close it will use melee. The giant scarab is also capable of mystically poisoning you, which starts at 98 damage.
One strategy involves luring the giant scarab so it corners you, out of the range of any Scarab Mages summoned. You can then use Protect From Melee to mostly protect yourself from the giant scarab's attacks. You may also choose to kill the minions first, though you will have to be careful of the giant scarab's ranged attacks. If you bring dragonhide armor (red or better recommended) you may choose to ignore the mage and focus on the giant scarab.
After defeating the giant scarab, its minions will disappear and Osman will reappear, having hidden in the shadows to protect himself. He thanks you for saving him and departs. Don't forget to pick up Keris where the giant scarab was killed, then return to the High Priest. He will tell you that the Sophanites in Menaphos have returned, and the quest is complete.