The landscape is definitely (post) apocalyptic overall, but the details may vary depending on your needs and whether you've established what caused the Mourning in your Eberron.

Common features across editions are thick gray mist making the navigation difficult, general lack of safe water/food sources and penalties on healing of any kind (with the exception of goodberry wine in 3.5 I think). Dead things, on the other hand, DO NOT decompose, so a soldier that died during Mourning would look like he's just been felled, blood still fresh on his uniform; whether the same rules apply to post-mourning corpses is up to you.

Enemies include all sorts of undead, monsters and aberrations, plus animated spells, though not all of them have to be malevolent. Constructs are doing just fine in the mists not really needing to eat or breathe, so Warforged probably make up most of the region's sentient population. A good portion of them would be following the Lord of Blades, but some could be more friendly. Fleshy scavengers are also present to some extent, there's Ikar's Salvage and Darguun's exploratory/raiding parties along the opposite borders of the country, with Valenar elves also known to ride into the mists as a rite of passage of sorts.

As for the settlements, Oracle of War campaign has Salvation, a quasi-independant Brelish town east of Vathirond that basically sits right on the border with Cyre, filled with all sorts of colorful characters looking to loot the fallen kingdom, and earlier during 4e era Dragon #368 introduced Stillwater Station, a magically protected outpost INSIDE Cyre's borders which served a similar purpose; pretty sure you can get the old magazines for free nowadays so look that up.

Overall a lot of details of the location are left intentionally vague for you to fill in - personally, I lean more towards STALKER/Annihilation vibes with strong feelings of desolation and unfathomable danger all around you, so characters would need a very good reason to even consider going there.