Chapter 26: Battle before Dawn
To unlock chapter 26X, Nino has to talk to Jaffar before the chapter is finished.
To do everything, and get all the treasures, you have to move fast. You'll need at least one thief to get all the treasures.
First, split your units into two groups to move left and right, the group moving left should have two high movement units and a thief. The group moving right should have at least one high movement unit and Eliwood.
The left group: Open the door with the thief or a door key and rush one of your high movement unit down to rescue Jaffar and bring him to the right so that Nino can talk to him and unlock the sidequest. The other high movement unit should carry the thief and run for the locked door. By the time you reach there, an enemy thief should have appeared, so you're just in time to kill him, perventing him from stealing the chests. Then either take the treasures with your thief or a chest key. After the thief has finished his job on this side, use the high movement unit to carry the thief and bring him to the room with treasure chests on the right side. You'll see why later.
The right group: Open the door with a thief or a door key, or wait until the enemy opens the door, then move your high movement unit down to the lock door. By the time you're half way there, a thief should already oped the door and maybe stole something already. Move as fast as you can so your unit can block the enemy thief from escaping, and when your own thief arrives, steal the item and finish him off. Move Eliwood next to Nino so you can get a really good mage on your team, then move one of your strong units to protect Zephiel.
Once you finish this chapter, you'll get your second Heaven Seal.