Solution: Finding the Demon
Answer: IT WEARS PRADA

Written by Tan Chor Kiang, Eunice Wong, and Adib Surani

This is a Masyu puzzle, as hinted to by the phrase "evil influence".

The rows can be filled in from the round answers by using the black circles as word-breaks; no two rows have the same pattern.

Once this is done, there is a unique solution to the Masyu, treating the O's as white circles:

demon

The remaining letters not crossed by the path spell out the final answer IT WEARS PRADA.

Author's notes (Adib)

This grid was constructed by a computer. The main requirements were:

  1. Every row contributes at least one letter to the final answer.
  2. Having any 9 of 10 rows results in a non-unique solution (which necessarily implies that every answer must have an O).
  3. Every row had at least three candidate answers.

There turned out to be quite a number of possible grids, so I went with this one which maximises the number of possible solutions in the initial grid, i.e. when no white circles are known. (I can't remember the exact number offhand, but I think it's in the order of a million).