Talk to Ailbeche again and he'll request that you get him a Giant Shell Bug, the bait he needs to go with his friend to fish up the big one.
Head to the Crawlers' Nest. Go to the center section and then north at the top of I-9 on the first map. This will take you onto the second map, the one with the other exit to Rolanberry Fields. On this map, go to the west side of H-9 and you'll find a ??? click it to spawn the NM Dreadbug. The Dreadbug will drop a Giant Shell Bug. Alternatively, you can often find Giant Shell Bugs on the AH.
Head to Castle Oztroja. To get to the pool with the fish, go on the first map to the map exit at J-8, through the door and on to the second map. On the second map, head northwest to the map exit at G-7. On the tiny third map, go due north through the map exit at I-7. This will take you to the forth map, which has the pool with the big one. Walk along the western side of the map and make your way into the center section at H-8. Here you will find a pool with a few ooze inside it, and various Yagudo wandering around. Clear the room.
When ready, get out your fishing pole, equip the Giant Shell Bug, and fish in the pool.
Some points of note about fishing:
It doesn't matter who does the fishing, you or someone else in the group.
You can fish it up with absolutely no skill in fishing, but it may take a while.
The Yagudo in the room respawn about every 8 minutes.
The number of times you have to fish before you land it can vary from 3 to over 20.
If you have never fished before please practice so that you get used to the system. It is very easy to fish but if you've never tried you might lose your Giant Shell Bug. Practice with Little Worm as your bait at any pond where you can fish, such as the moat in Northern San d'Oria.
Your line can break and you will lose your bait, so bring extra, just in case.
After killing Odontotyrannus, it turns into a Rare/Ex item of the same name, which you take back to San d'Oria and trade to Ailbeche, who tells you to go get it cleaned by an expert fisherman in Selbina.
Head into the Chateau d'Oraguille from Northern San d'Oria and go to H-7 in the gardens, where you will find the door to the Prince Royal's Room. Select the door for a cutscene with Prince Trion and to complete the quest.
Client: Ailbeche (Parade Grounds, Northern San d'Oria) Summary:
All fishermen dream of hooking the big one. Catch him and obtain the treasure in his belly.