RTF is a global modification that replaces all towns in the game.

I don't think panthers live in deserts :) And this is closer to orcs.

You could write that they are warriors from the islands, or that they went to the islands to undergo some kind of trial and earn the title of panther warrior, although perhaps they are warriors from the islands after all. The increase is significant, and the trial implies that it should be undertaken by a special warrior, one could say, an chosen one. You could even write the entire alternative upgrade on the islands.