"I am the end of Chaos, and of Order, depending upon how you view me. I mark a division. Beyond me other rules apply." Ygg, 'The Courts of Chaos', Chapter 7
Ygg is a sentient talking tree, quite small, gnarled and ancient-looking, of no known species, which grows roughly halfway between Amber and Chaos. It claims to have been planted by Oberon, and to be a boundary marker. Generally it seems not to have much to say.
Regardless of the route one takes through shadow, Ygg (or the shadow it is in) always seems to be there, roughly halfway between Amber and Chaos.
The voice of Ygg is not human to say the least. It is "something like a voice from within it," as Corwin puts it (in 'The Courts of Chaos', Chapter 7). It can sense the presence of people, and powers such as the Jewel of Judgement.
The tree of Ygg sits in a fog-bound bowl-shaped valley. Part-way down the slope one descends as one travels away from Amber, just at the upper edge of the fog filling the valley, is the ancient-looking Tree of Ygg itself. The sky over the valley is streaked with red, the colour sometimes turning to green, or yellow. This is somewhat similar to the sky over the Courts of Chaos.
Below Ygg, in the fog, the going is rough, then becomes clearer, with "Rocks. A stagnant pool, some small dreary trees festooned with ropes of moss, a smell of decay." It was here that Corwin met the dark bird Hugi which accompanied him for a while. The befogged marshy area continues for a long while. Snatches of music come out of the fog, though their direction cannot be determined. They are "slow, and somewhat stately, produced by a steel-stringed instrument." (Quotes from 'The Courts of Chaos', Chapter 7)
Just to one side of the route Corwin took is a bald, stone-textured milk-pale-skinned giant with dark eyes, buried to the shoulders in the swamp, though he does not want to be freed. He tried unsuccessfully to persuade Corwin to let the universe as a whole die. Hugi called him the Head.
In the The Last Enemy game game, Ygg is also a self-aware Trump monitoring device, placed by Oberon where the structure of shadow is best for Trumps - midway between Amber and Chaos, in the centre of shadow. Unfortunately, Oberon is no longer there for it to report to, of course.
Although it is not obviously a Trump item or item of power, it can scry and spy on Trumps, though it cannot jam or disrupt them. It has Trump Memory of all the elders, and can gain a Trump memory impression by the scrying of people using Trumps, seeing the Trump of a person, or seeing a person itself. It also has limited predictive powers, using Trump readings.
It was for this reason that it gave Oberon a branch of itself to plant because it saw what he might soon do, and part of its programming was to monitor any such things.
Lastly, Ygg has a number of birds (such as Hugi, who travelled with Corwin) that it uses as roving eyes, thieves of Trumps, and so on.