An "ominous message" refers to a communication or statement that gives a feeling of something bad or threatening about to happen. It typically signifies a warning or indication of an impending negative event. Full definition
This short, ominous message accompanied by one of several quotes from classic survivalist novels was all the instruction I received before I was thrown headfirst into the cold and unforgiving landscape of The Long Dark, Hinterland Studio's new wilderness survival simulator currently in alpha testing. (thatvideogameblog.com)
Then an ominous message from the attacker pops up, demanding a ransom be paid in order to unlock the computer or decrypt the data. (cnbc.com)
The kidnapping of two bishops sends an ominous message to Syria's Christians, and Prodromou is outraged that the U.S. did not see fit to introduce a Security Council resolution condemning the kidnapping. (firstthings.com)