Learning the ZODIAC summon spell.
It is possible to learn the summon spell "Zodiac" that is cast by Elidibs, the last boss of the Deep Dungeon. To do this Elidibs must cast Zodiac on a Summoner and the Summoner must survive. To do this use the Knight ability "Mind Break" and other spells to lower Elidibs Magic Attack Power. Hopefully you will survive. Once you know it, you can cast it on other summoners in your group to teach it to them too.
What is the Deep Dungeon.
The Deep Dungeon is a subquest that becomes accessible after the St. Murond Holy Place battles. When you walk up to Warjilis Trade City, you will trigger a scene at the bar. When you leave, a path will open to an island in the East. This is the beginning of the Deep Dungeon subquest. It consists of 10 dark and violent levels: Nogias, Terminate, Delta, Valkyries, Mlapan, Tiger, Bride, Voyage, Horror and End. Each level will open up after you complete the previous battle and find the exit. If you don't find the exit the next level will not become availble. If you replay the same Dungeon, the monsters will be differnt. This is also a great place to Poach. The variety of enemies is large. The nearest Fur Shop is in Warjilis. You can go back and forth without having random battles.
The Secrets of the Deep Dungeon.
The Deep Dungeon is FULL of powerful items. To get them you must do some work. Put the "Move-Find" ability of the Chemist on someone with low Bravery. Rafa is a good choice. If you are lucky you will find great stuff. Sometimes an item should be in a spot but you get something else, all you can do is reset and try again.