A longer fear duration definitely will make Nocturne viable and a little too strong if you don't tone something down a bit with it. Something that would directly co-aside to something like this is his ultimate, so I recommend you at least tone back his ultimate's damage to compensate for a heavy buff like this. Something like 100/175/250(+100%bonus AD) from 150/250/350(+120%bonus AD) physical damage should be good, because a fear duration increase would grant Nocturne an INSANE amount of power on its own, other things would probably need to be toned back too.
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