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warlords: señores de la guerra
unfortunately it doesn't sound as good if you're just talking about ONE warlord (it's Carlos Castaño in the photo). I haven't heard this term widely used in Colombia, but it's in the title of one of the best books on the paramilitaries in Colombia:
Los señores de la guerra: de paramilitares, mafiosos y autodefensas en Colombia by Gustavo Duncan