I am currently trying to create a very low fantasy setting, with the most scientific plausibility I can, in which, with technology and "human" development being akin to those of Earth's medieval period, but, of course, with some differences. Well, none of that matters very much for my question, but I am just trying to provide some context. Do note that, in this setting, all laws of Physics as we know it apply, with a single "catch": the base elements of nature (earth, water, fire...) can more or less be controlled, though that can only be achieved through years of training and practice (and doesn't work quite like normal high-fantasy magic: there are no "elemental blasts" and nothing like that).
Well, in the middle of continent upon which I plan to make my story happen (which is somewhat like Pangea, in the sense that it is the only continent, but comprised of several tectonic plates), I intend to have some kind of grasslands (more akin to steppes, but not so arid), more or less surrounded by mountains (created by convergent plate boundaries). But I was thinking about making these grasslands turn into some kind of very harsh landscape, with little to no water and very sparse vegetation, arid but not desertic, and preferably with terrible weather (some kind of "dry thunderstorms", perhaps?).
So I was hoping someone in this very helpful website could enlighten me: what kind of natural disaster could make that change, but without impacting significantly the climate patterns of the rest of the continent? Massive volcanic eruptions could be an option, but how could I make their effects occur only in that area and not globally?
I thank you in advance for your answers.