If is plausible, what are the most reasonable factors to bring to bear?
I have a large continent where a number of races/cultures are present in the present day, but I want the one in the south to have arrived a number of centuries ago after a cataclysm destroyed their homeland. This southern area has some desert and is quite temperate compared to the rest of the continent north of it, but I need a plausible reason to keep the other races/cultures from having expanded down there.
They are all humanoid races, somewhat like standard Elves, Dwarves, Orcs, Celts etc... but they are all far more "human" than any of those stereotypes such that their cultures are largely what separate them, though they do have some slight physiological differences.
I don't want a major conflict in the south when this new race/culture arrives because there were only a limited number of ships that escaped their homeland and they were sailing for long enough that their supplies were stretched to the point of breaking and they were extremely lucky to stumble across the land they did. They do encounter the race/culture to the west but the language barrier and the lack of anything much to trade means they leave pretty much empty handed and largely un-harrassed. Scout ships find uninhabited, suitable lands to the south and then they split off into different groups forming their own colonies across the southern part of the continent and eventually become an empire of different states ruled centrally from the imperial capital.
I'm happy with all of it bar the fact that it seems way too convenient that the southern lands are uninhabited and they can just waltz in and start setting up their new empire. I've thought about having a precursor race/culture that dies out leaving small clusters of survivors that become the races/cultures of the present day and either fear, tradition, superstition or risk of disease keep them from venturing too far south. E.g. Perhaps the source of their downfall was in the south, but I'm not sure if that makes any sense or if I'm just throwing additional complexity in where it isn't needed.
Any suggestions greatly appreciated.