Dantarian is an optional boss found in the left library in Alexandria Castle. Examine the bottom-most shelf, trigger the question mark, and choose the first option twice to fight it. You'll only have two chances to battle Dantarian: one in the middle of Disc Two and one at the beginning of Disc Three. Obviously, you stand a much better chance of winning during the second encounter.

There's a monster at the end of this book

   Dantarian spends most of the battle hiding inside a book. Every time you attack it, the book turns forward a number of pages equal to the damage you do. When the book turns to or past page 150, it opens up to reveal the demon. You can hit it once before the book closes and everything resets; use your strongest attack (probably Zidane's normal attack).

   Unfortunately, Dantarian has some powerful moves up its own bookmark. In addition to using a page-spewing magic attack against the whole party, Dantarian constantly casts Death Sentence. You'll want to keep reviving dead characters as quickly as possible, as there's safety in numbers -- Dantarian can't Death Sentence everyone at once. Equipping the anti-poison skill is also a smart move, as Dantarian uses poison spells.

   Defeating Dantarian earns you a hefty 30 AP, as well as a pair of Hermes Shoes. You can also steal Demon's Mail and a decently-powerful fork from it.