Equip Rosa with the CrossBow and whatever you took from Tellah. Leave town. The way to Fabul is northeast of Damcyan.

Go in the first door you see. It leads to a savespot and some items. In the next area, you find some white arrows. I suggest that you save those until you get the twins. Then you see Yang kickin' some butt. Then you join in the fight. Just beat the crap out of it, and it will turn into a bunch of little ones. After the fight, Yang officially joins your team. Then leave the mountain by the door that was behind Yang. Fabul is east of the mountain. Go to the Inn and rest, and then go to the weapons/armors dealer. Buy your favorite claw for Yang, and buy all of the black stuff for Cecil. Then go up the middle set of stairs.

At this time it is a good idea to unequip Rosa. Go to the King's Chamber. After some dialog, you go down to the entrance of the castle, and start getting into a long line of fights. All the while getting closer to the Crystal Room. You go into the Crystal Room, and Kain shows up. He doesn't look like he wants to be friends, though. You will more that likely lose. After the battle, Golbez makes his first appearance. Whole buncha stuff happens. Leave the Crystal Room, and go to the little button on the right side of the King's Chamber. Go through the door and get a Black shield.