By Witness

The Riddle
The Trial of Gallus area is part easter-egg, part credits, part parody of Diablo II's Secret Cow Level (also known as the "Moo Moo Farm"). ToG is not an official quest on the Utraean Peninsula map (i.e. nothing in your quest journal indicates that it would be), but a book entitled the "Trial of Gallus" (gallus is Latin for chicken) can be picked up in the area below Hiroth's Utraean Circle on your way towards completing the final quest in the multiplayer map. It contains a riddle:

Climb to the sky on the sunbaked mesa
Give to the moon with the treble stones
Place down the eye of the crystal temple
Gift of the steel from a virgin hand
The final piece within your grasp
A leap of faith convenes his trial.

The rest of this article is also dedicated to explaining how to get the "quest" items, chock full of spoilery goodness, plus some tips on how to make the feathers fly once you're there.

The "Quest" Items

Getting the Fury's Eye
From Jeriah's Cabin (which is conveniently a starting location in the Kingdom of Ehb map), go South West. Follow the path to a cave that has crystals in it (unnamed area). Continue following the path within (it leads to the Dark Forest), but once you reach a winding long bridge whose beginning is framed with a rectangular arch, instead of crossing, take a right turn (NW, more west than north) onto a side path and you'll find some stairs to a hidden dungeon further down (the Trog Temple Dungeon). Drop some gold or useless items to hold down all the pressure plates (and keep going down the stairs that are thus unlocked). The Fury's Eye is in a chest in a side room (with a pedestal holding 152 gold) at the bottom level.

Getting the Starter Knife
For this, you'll either need two computers that you can use to make two characters join the same game, or a friend to hold the item for you. Create a new character and transfer the equipped knife to the character you will be using to go to the chicken level.

Getting the Trial of Gallus Book
For this, you will need to complete most of the Utraean Circle quest in the Utraean Peninsula map. Once you can take the elevator to the catacombs, do so and you'll find the book lying on the ground nearby.

The Forgotten Mesa
You'd think that "sun-baked mesa" would refer to the Mesa Desert near Grescal, but it actually refers to a hidden area called the Forgotten Mesa near Quillrabe.

A Leap of Faith
Once you get to the Forgotten Mesa, you'll travel through a barren land and up several elevators. Once you reach the end of the line, you will see three pressure plates and an empty elevator platform. In order to activate the elevator, you will have to drop the "quest" items on the pressure plates. The drop order doesn't matter (i.e. you don't have to drop them in the order stated in the Trial of Gallus book), but they have to be put on the correct pressure plates:

Once you've dropped the goods, the items will vanish, and a platform will descend. Ride it up to the top, and walk across the invisible bridge ("leap of faith") to reach another elevator that will take you down to the Trial of Gallus.

The Opposition
Lots of big chickens (~30, named after members of the Dungeon Siege development team), a big rooster (Chris Taylor), and an evil reincarnation of Norick (that guy who dies on the bridge at the beginning of the single player campaign), the Colonel. The chickens yield lots of loot, but all sport high defense, lots of hitpoints, hit hard, and come in droves.

Are You Ready?
How do you know when you're ready to take on a whole team of fowl incarnations of the GPG staff? Well, let's first take a look at their attributes in (note that health scales with the number of players and these hp values would seen in 4 player games):

Regular/Normal

Veteran/Normal

Elite/Normal

*The Chris Taylor chicken uses a spell-based damage attack called Chicken Fry.

A few tips and notes:

Giving Colonel Norick Some of His Own Medicine
It's all in this cheezy little lvl 50 Nature spell called "Shared Pain" (reflects 50% of damage taken). I do not suggest trying this at home unless you have at least the following (for Regular difficulty):

Simply cast Shared Pain on the tank (who will be the center of Colonel Norick's attention) and let Norick beat himself to a bloody pulp. Shared Pain should stack duration, and the process will take about 10-15 minutes (for 35,000 hp).

Variations on this process include using Pain Reflux (reflects 30% of damage taken) and buffing with health regeneration spells.

What You Get For Your Troubles
You'll get more Rares and More Rares than you can fit on one screen. Most of the armor and weapons have level steep requirements of 50-77 in a particular stat. Rare and More Rare rings and amulets may also drop. This is where Dragon Wings reportedly drop.

According to the drop configurations of the chicken templates, Veteran and Elite chickens drop the same kind of loot, with the only variation being the amount of gold dropped.

In Regular difficulty,

The Chris Taylor chicken can drop:

The other chickens can drop:

In Veteran/Elite difficulty,

The Chris Taylor chicken can drop:

The other chickens can drop:

Colonel Norick drops the Chicken Gun, whose stats are:

200-350 Damage
25 Str Requirement
25 Dex Requirement
Faster Attack Speed
Two Handed Weapon
10 Meter Range

It has serious accuracy problems for any targets farther than 5 meters, but the ensuing hilarity sometimes makes up for it.