The phrase "sinister secret" refers to a hidden or concealed piece of information that is harmful or evil in nature. It suggests that something is being kept hidden with the potential to cause harm or be morally wrong. Full definition
Explaining how their new Suspiria compares to the 1977 original, an Italian horror film about a young ballerina who uncovers sinister secrets at a prestigious dance school, Swinton told Digital Spy: «It's a cover. (miagoth.com)