The phrase "ancient evil" refers to a very old and wicked force or entity that has existed for a long time. It often suggests something powerful, mysterious, and dangerous that has been around since ancient times. Full definition
A powerful ancient evil has arisen, and the war to save humanity has begun! (gameanyone.com)
These two alone must defeat an all - consuming ancient evil. (bookbrowse.com)
Speaking of saving the universe from ancient evils, Mass Effect 3 brings back Commander Shepard to resume the battle against the machine race of Reapers. (nonfictiongaming.com)