You'll quickly notice a Nu walking back and forth with his back to you in this Area. Offer to scratch his back to learn the Nu's scratch-point, which will let you get a Magic Tab in Kajar.
You'll find Schala and Janus in the northeast room. After that, follow Schala to the central path and watch her open the sealed door. That crest should look familiar. Head to the Mammon Machine in the western room and charge up the Pendant, and you can open the door as well. You won't be able to leave after charging the Pendant, so follow Schala through.
Inside, you'll be attacked by a Golem. You don't actually have to win the fight, but you can if you're careful. Golem will changes its attack routine to match your last attack's type. Its Physical attacks are devastating, so stick to magic if possible. Afterwards, you'll be banished from the Dark Ages, but don't worry—you'll be back. And now you can open all those sealed chests and doors!
When you return here after the Mountain of Woe, you'll face Dalton. He is a pretty weak Boss, so you shouldn't have any trouble with him. All he does is counter every attack by reducing the attacker's HP by half. He has one final attack that can deal upwards of 100 damage to every party member, so keep your HP reasonably high.