Final Fantasy Tactics Walkthrough

(5,10): Elixir
(5,12): Chirijiraden
(7,10): Chaos Blade
(7,12): Elixir

The origin is in the corner where the stairs begin to go up.

This can be a difficult battle because the Reavers are really tough. Bring along someone who knows Holy, or someone powerful like Orlandeau, and be very cautious while going up the stairs. Focus on one Reaver at a time but don't get too close to the others, or you might get petrified or frogged. Once you've made your way up the stairs, you'll face Elidibus, provided he hasn't come down after you yet. He can be difficult, but if you stay spread out and continuously attack him, he'll eventually fall. The battles on the way here are exquisite for grinding up your party and also getting them some high-quality equipment, so by the time you get here you should be able to withstand whatever is coming your way.

One of the Reavers is a special one! Its name is Byblos and it will fight as a guest on your side. If it stays alive through the entire fight, it will offer to join you permanently. At this point in the game, it's maybe not the most useful addition, but it's a symbol that you completed the dungeon at least once!

If you can get Elidibus to cast Zodiark on a Summoner, and the Summoner survives it, the Summoner will learn the spell. This sounds easy but Zodiark is an intensely powerful spell. Bumping up the character's magic defense through equipment or magic can help, as can reducing the character's faith via Beowulf or Orator skills. This is the only place to learn it in the game from an enemy - if you really want other characters to have it, you can have the original character get charmed and then cast it on the rest of your summoners, but, do you really want to jump through those hoops? No. You do not.