Certains permettent de faire d'une victime une marionnette obéissante. Une pratique interdite, s’il en est.
While I roughly understand the meaning of this expression, I find the definition in dictionaries somewhat lacking.
Given that there is no accord between the feminine noun "pratique" and "il", I assume this "il" is actually an impersonal, placeholder subject and the entire expression is synonymous with "s'il en existe une", for instance.
I wonder if this expression means something along the lines of:
If there were such a thing as a forbidden practice, this would be one.
A couple of example sentences with "s’il en est" would not go amiss!