This design focuses on the Mouth of Sauron's mount, an other-worldly horse with flames burning in its eyes and nostrils
“At its head there rode a tall and evil shape, mounted on a black horse, if horse it was; for it was huge and hideous, and its face was a frightful mask, more like a skull than a living head, and in the sockets of its eyes and in its nostrils there burned a flame. The rider was robed all in black, and black was his lofty helm”
—Excerpt from Lord of the Rings, book 5 chapter 10, The Black Gate Opens
An alternate design for this chapter focus on the gate itself, appearing as a gaping mouth.
“The two vast iron doors of the Black Gate under its frowning arch were fast closed. Upon the battlement nothing could be seen. All was silent but watchful.”
“There came a long rolling of great drums like thunder in the mountains, and then a braying of horns that shook the very stones and stunned men's ears. And thereupon the door of the Black Gate was thrown open with a great clang, and out of it there came an embassy from the Dark Tower.”
—Excerpts from Lord of the Rings, book 5 chapter 10, The Black Gate Opens