How do you find out who is behind a black magic spell?
If you are intending to “send the spell back” or retaliate O suggest you re-read the Rule of Three, sending it back is unnecessary as the gods/fates/universe have that well in hand. As to the retaliation… just don’t go there. Think long and hard on your motivations for searching out this “person”, two wrongs don’t make a right, learn to protect yourself from further such incidences and trust to the gods for the rest.