Give the jaw to Fiun Rehlyun in Fiun Outpost at 95.9N 57.0W. This flags you to enter the quest dungeons.
There is a portal near the end that many people have a tie to, have someone summon it for you if possible, often called Bellas Portal.
If you can't find someone with a tie you'll need to traverse several dungeons (see maps).
At the end of the final dungeon is Sir Bellas, he drops Blank Augmentation Gems that you can then give to any Augmentation Trainer to get a gem that when used collects the xp and gives you the augmentation. If you make a mistake before using the gem, you can give it to any other Trainer for an exchange. See Augmentations for a list of available augmentations, what they do, and where the NPCs are located.
SIr Bellas is a Knight, and equally vulnerable to all elements. His weapon is also hollow and so ignores Item Magic. He buffs himself randomly against magic if vulned/imperilled (counterbuffing). Use your best weapon against him (I prefer Silvaran Weapons). If you can Imperil and Vuln him, a Knight SLaying Sword (the replica Sword of Bellenesse) will kill him quickly, but most any AR or CS weapon will do.
KIlling Sir Bellas also earns you one of the notes that can start the Assassin's Roost Quest. He no longer has an uber loot profile, but he does drop boss-level quantities of loot. Without exceptional melee defense he is exceptionally difficult to tank.
Fiun Rehlyun tells you, "You wish to improve yourself? Enhance those abilities which might otherwise languish? Return to me the Jaw of the Insatiable. Prove yourself."
You give Fiun Rehlyun Insatiable Eater Jaw.
Fiun Rehlyun tells you, "One who obtains such as this is truly worthy of that which we would teach. The highest peak of the deadliest isle contains that which you seek."
Fiun Rehlyun closes its eyes and whispers. You feel a subtle vibration within your chest for but a moment, then it is gone.
