Hades

From Manual of the Planes (3e):

It is where evil springs eternal.

It is a plane of endless apathy and despair.

It is the great battlefield of the Blood War.

Hades sits at the nadir of the lower planes, halfway between two races of fiends each bent on the other's annihilation. Thus, it often sees its gray plains darkened by vast armies of demons battling equally vast armies of devils who neither ask nor give quarter. If any plane defines the nature of true evil, it is the Gray Waste.

In the Gray Waste of Hades, pure undiluted evil acts as a powerful spiritual force that drags all creatures down. Here, even the consuming rage of the Abyss and the devious plotting of the Nine Hells are subjugated to hopelessness. Apathy and despair seep into everything at the pole of evil. Hades slowly kills a visitor's dreams and desires, leaving the withered husk of what used to be a fiery spirit. Spend enough time in Hades, and visitors give up on things that used to matter, eventually giving in to total apathy.

Hades has three layers called "glooms." Uncaring malevolence that slowly crushes the spirit permeates each gloom.

The glooms of the Gray Waste are just that: dull gray lands. The earth is gray, the sky is gray, and the petitioners are gray. Color is foreign here, as if vision itself is subverted. When visitors step into the plane, everything goes from color to white, black, or gray. There is no sun, no moon, and no stars above—just a bleak gray radiance emanating from the sky.

This grayness affects more than vision; it is a spiritual grayness. It reaches into the hearts of all who spend time in Hades. Those who spend more time here than they should, such as all the petitioners, are devoid of feeling. They don't laugh, don't cry, and just don't care. All they do is despair, their hope gone and never to return.

Both the entrapping trait of Hades and the spiritual sickness called "the grays" are manifestations of the grayness of Hades.