Dull they seemed and harsh in the heavy air, braying ominously.
Behind it there hung a vast shadow, ominous as a thunder-cloud, the veils of Barad-dur that was reared far way upon a long spur of the Ashen Mountains thrust down from the North.
As Sam stood there, even though the Ring was not on him but hanging by its chain about his neck, he felt himself enlarged, as if he were robed in a huge distorted shadow of himself, a vast and ominous threat halted upon the walls of Mordor.
The Mountain standing ominous and alone had looked taller than it was.
Far away now rising towards the South the sun, piercing the smokes and haze, burned ominous, a dull bleared disc of red; but all Mordor lay about the Mountain like a dead land, silent, shadow-folded, waiting for some dreadful stroke.
There are no more uses of "ominous" in the book.
Show samples from other sources
I’m beginning to hear ominous rumblings of discontent.
With the clouds looking so ominous, I’m afraid there will be a bad storm.