How would you translate “dark project” into Latin, as in a secret project? I know tenebris means dark, but I’m having trouble finding the word for “project.”
I would suggest opus arcanum meaning "secret task/secret work."
Some abridged definitions from Lewis & Short:
A. In gen., work, labor (cf.: “labor, ars, opera); B. Esp. 1. Work, art, workmanship 2. Of agricultural labor 4. Of literary labor II. Transf., a work that has been done or made.
Arcanum (neuter to match the gender of opus):
II. Hidden, concealed, secret, private (class., although very rare in Cic.) — “Esp., in the lang. of religion, of things sacred and incommunicable; “and of secret, mysterious usages: sacra,”; B. Spec., a sacred secret, a mystery