"Unspeakable evil" refers to an extremely wicked or horrifying act that is too terrible or disturbing to express or describe properly using words. Full definition
If Cafardi's is a counsel of despair and surrender in the face of unspeakable evil, Douglas Kmiec finally came around to adopting the pro-choice position while continuing to call himself pro-life. (faith.org.uk)
Now, as the fate of all mankind hangs in the balance, Rick O'Connell (Fraser) and his wife Evie (Rachel Weisz) embark on a daring, desperate race to save their son — and the world — from unspeakable evil. (uphe.com)
In this 1927 classic tale of cosmic horror, a small Massachusetts farming community faces unspeakable evil from the outer reaches of the universe. (scientificamerican.com)