Straight away you unlock the ability to change characters. I personally stuck with Nien, but Sevan can use her neat magic spell in the overworld too.
In here you'll find a lot of new enemies. This is one of the more boring ones, a Spider.
Throughout this dungeon you'll "randomly" find items that go to this screen. Most of them are completely useless and just jokes at items in other games, one item will be useful later...
Very quickly you'll find a super helpful unlock that gives you a health bar! Head up from that chest.
Just wonder around this place and fight everything you find. Not only will you eventually find your way, but you'll be swimming in cash (for that Fast DVD loader) and you won't miss the important item.
After a bit there is a treasure chest with a Minimap in it. A very useful unlock you don't want to miss.
Eventually you'll reach a path with a small off-path. There is an important save here, grab it and continue down a bit.
Once you beat all the enemies down here, I'd strongly suggest heading back to save again. That big skull on the Minimap doesn't look pleasant.
Just as you thought, that skull was a Boss, and not an easy one either. He can hit pretty hard and has pretty good health, so you won't be able to tank him. Instead you're in for a long battle of hit and run. Take only clean shots at him and protect your health.
If you're health gets too low, wait for him to summon a bunch of bats. Like normal bats, these sometimes drop Health pickups when killed.
After that long battle of back and forth, you should eventually beat him. The gate directly North of you will now be open. (Don't forget to save).
Head up to find the second and last piece of the Amulet! Conveniently you also unlock a portal directly to town.
Now that you're in town, stock up with all the Potions and Phoenix Downs you can, the next part isn't an easy one! (Also grab that Fast Dvd Player to get rid of those annoying load times).
After you stock up, head over the Ate to find out you can now enter the Black Citadel. It's completely east of the Village from the back exit.