I have a region in my homebrew D&D setting that used to be grassland/fertile farmland, and the breadbasket of the elven empire, until some wizards did a collective magical ritual that permanently altered the afterlife, made necromancy spells possible for arcane casters to use (they used to be solely the purview of the gods and their followers), and turned all of the casters into liches except one who became a death god.
As a side effect of that spell, I want the region that they cast the spell in to become a desert. Would it work to have it just be that lingering necromantic power is killing the plants for several decades? Would that be enough to turn fertile farmland into desert? (If not, I could also finagle an angry sun-goddess making things worse, but I'd only like to do that if her involvement is necessary to explain the transition.) What happens to grassland if the plants stop growing in it? I know cutting down trees can seriously ruin the soil of a rainforest, but does removing too much grass from a grassland have similar effects?
Also, the specific city they did the spell in was on the side of a lake. What would be plausible to happen to that lake? Would it dry up? Would it get clogged up by loose dirt being blown away in the wind? Or would it still be a lake, but just eerily lifeless now?