The Master in Fallout 1 was probably the most terrifying thing I've faced. Before you even reach the boss your character starts to die from hallucinations induced by the FEV emissions. And then after a drawn out conversation trying to convince him to stop producing super mutants (And you hoping to god he complies so you don't have to fight this monstrosity) you're pitted against him in a 1v1, and you suddenly realise you can't hit him with anything because he's reduced your perception to 2.
Oh, and pretty much every fast moving mutant in the STALKER franchise.
Oh, and pretty much every fast moving mutant in the STALKER franchise.