Here's how you get to the secret tunnel that leads to Cerberus and Lord Agramon in the Labyrinth.
From the top level of the hub, take the southwest tunnel to the stairs as shown by the purple arrow. Go down two flights.
You're now on the second level. Head northeast.
As you walk down the sloping passage to the northeast, your map will change to level 3. The entrance to the secret tunnel is near the end of this passage on your right side. A mob named Bazil may be there.
Click the protruding stone in the wall.
The secret entrance will open. Head in and continue down the path until you reach Cerberus. When you kill him, a gate will open to Lord Agramon.