How long ago was this war?
I think I would do a mix of bad lands and swamp/bog/marsh lands. Reason for this, magic in warfare, you would have the trenches of WW1 but seige equipment falling apart here and there. The land would be slowing going back to wilderness, bushes and small stands of trees. Water would be settle in low areas, making the bogs. High ground would be blasted and harden, protected by age old magic, very little will grow on them.
Monsters, your undead, ghouls and such but also constructions could be still roaming, obeying their last orders. Then you have your aggressive plants, just looking for fresh blood and iron rich soils. Also, insects and spiders, one brings the other and with loose magic; they get big!