City of Suffering/Judgement

The most info on the Gray Waste is going to come from the Planes of Conflict box set, with some additional relevant information in the 'Hellbound: The Blood War' box set, and 'Faces of Evil: The Fiends'.

The largest problems to face anyone moving through Oinos are as follows:
1) The Gray Wasting
2) Disease
3) The Blood War

1) The spiritual wasting of the Gray Waste is something like the multiverse's sinkhole of positive emotions. The plane sucks passion, happiness, drive, motivation, altruism, care... leaving only blind, pointless, abject misery in its wake. PCs first experiencing the plane are going to be shocked by the exposure like being suddenly plunged into ice-cold water; it's a harrowing experience and after the shock while you can adapt and steel yourself against it, it will grow and eventually you'll start to fall pray to it the longer you stay exposed. The 3e Manual of the Planes gives the game mechanics to handle this.

2) Outside of the spiritual malaise, the layer of Oinos is crawling with disease. The soil, water, and air are laced with it, and frankly sickness will manifest on its own without an apparent source like the layer of the plane were possessed of its own perverse immune system, seeking out and poisoning anything that steps upon it.

3) Oinos is the primary battlefield of the Blood War, and the worst thing a traveller can have happen is to be caught between the various sides, or happened upon by scouts, or deserters, etc. They might kill you, eat you, torture you for fun, pressgang you into fighting, etc.

Beyond that, the layer is crawling with larvae, the Night Hags who collect and traffic in them, and wandering packs of Hordelings (larvae that refuse to succumb to the wasting of the plane and who metamorphose into a new, unique form by defining themselves by their own agony).

Then there's the yugoloths. More on them in my next post, along with various locations on Oinos.

There's a map of it in the Forgotten Realms Atlas by Karen Wynn Fonstad.