How can we get the achievement "Forbidden Knowledge"?

Question

How one can get the achievement "Forbidden Knowledge"?

I know it has some relation to 

Forbidden Fruit

Key to Knowledge 

Forbidden Tome

but I'm not sure how the Forbidden Fruit interacts here...

Do I have to eat the fruit before? Or after?

Answer 1

The answer is -  eat the fruit before 

Source (spoiler): "Obtained by using a Key to Knowledge on a Forbidden Tome. This won't work if you have never eaten a Forbidden Fruit."