In this passage, Father is referring to a powerful magical force that exists in the world, which he believes is not owned or controlled by humans or demons. He suggests that this force is merely borrowed or channeled through certain individuals and can be used for good or bad purposes, depending on the intention and actions of the person wielding it.
Father seems to be warning that this magic can be dangerous if it falls into the wrong hands and is misused. The phrase "lash back on him" suggests that this magic has the potential to cause harm to the person who wields it in a negative way. He implies that only a "superior man" who has good intentions and uses the magic responsibly need not fear any negative consequences.
An example of doing "harm in the hands of the wrong man" could be someone using this magic to harm others or to gain power and control over others for their own selfish ends. This could lead to a backlash or negative consequences, as suggested by Father.