Scripts

It can definitely be done using the if... then... command. I think it's possible to make it so that a specific monster B1 (a weak stack of 1 unit) is located in sector a1, then:
function Second()
if command_to_check_for_object_existence(B1) then
commands_of_the_second_function
end;
end;
Region trigger.
Maybe I'm wrong, but it seems to me to be the most advantageous option.