The best place for this sentence is between sentences 2 and 3. That is because of the connective used (in fact). This linking word is used to give more detail about what you have said, it is a discourse marker. What is more, it is generally placed in front position in a clause. However, in informal situations it can occur in final position.
You should place the sentence there because it specifies the terror felt by Gilgamesh.
Upon seeing the beast Humbaba, Gilgamesh himself became seized with <u>dread and terror</u>. In fact, Gilgamesh was so frightened that "terror flooded his muscles, his heart froze, his mouth went dry, his legs shook, and his feet were rooted to the ground".
He was so frightened that terror flooded his muscles, his heart froze, his mouth went dry, his legs shook, and his feet were rooted to the ground.
