Taar's Investigation - Act 1 Companion Side Quest
Location: Northern and Southern Greilyn Jungle
Closest Portal: N/A
Objective: Retrieve the Hak'u Ceremonial Blade and then defeat Garganturax
Reward: Random Loot & Gold
Taar's Investigation is a Side Quest in Act 1 which begins by speaking with Arianne who is next to the Eirulan Great Hall. This quest is Taar's Companion Quest (each character gets one in this game) and you'll need to have her in your party to accept this quest as well as when you go to turn in the quest.
For this quest you will first need to obtain the Hak'u Ceremonial Blade from the Northern Greilyn Jungle. When you teleport into the zone you'll be able to see the silver star on your map, it's to the northwest of the Northern Greilyn Jungle Teleporter. There will be a small bridge you can run across to a Hak'u Camp, the Hak'u Ceremonial Blade will be impaled on a deer in the center of the camp (pictured below).
With the Hak'u Ceremonial Blade in hand you'll want to go to the Dark Bone-Filled Cave which is directly south of the Eirulan Southern Gate. The fastest way here is to go south out of Eirulan past the Pet Merchant and ride the elevator down.
However, if you hate Eirulan and the platforms as much as me, you can also teleport of the Southern Greilyn Jungle and follow the path west. It'll be a slightly longer run with more enemies along the way but - no platforms.
In the back of the Dark Bone-Filled Cave you'll find the Garganturax enemy (pictured below). Unless you have the Hak'u Ceremonial Blade equipped your melee attacks will do no damage to it and you'll see "You need to have a special weapon equipped to affect this" in the chat log.
Spells and Powers will still damage this creature even though melee will not and it is possible to kill it without equipping the Hak'u Ceremonial Blade, it'll just take much longer. Once you've defeated the creature all that's left is to return to Eirulan and turn in the quest.