You want to look into the old books for info on the Battle of Hellfen. It was at the battle of HellFen that Nagash rose to power for the second time and came to destroy the realm of man, Karl Franz lead a great army to HellFen where the undeads superior numbers could not be brought to bear due to terrain. Getting past the fluff of the battle and into the fluff of Karl VS the undead. Karl Franz is written as a deus ex machina, meaning he will steam roll just about anything. Only Nagash was ever described as good enough to battle him, and Nagash held the upper hand for a long while in the battle. Only due to Karls divine blessing was he able to beat him and send Nagash back to his eternal sleep yet again.