The labyrinth is a relatively beginner level maze where the objective is to kill all the enemies within the labyrinth and report to Bishapo.
After reporting to Bishapo you will be returned to the starting map (where you got the quest to kill the monsters).
Then you can enter the boss room where you will fight a higher-level and stronger version of Mosqui (which now has an AoE shockwave attack), the boss from the Sewers in Mission 2. and Astaro, the boss from Mission 3.
Your objective in the boss room will be to kill Mosqui, killing Astaro will result in its respawn a short amount of time later (though both bosses will drop items when killed).
A map of the labyrinth: (Credit to the legendary DeadMachine)