As I see it, there are three moments in
The Illearth War where Hile Troy is plunged into complete despair. The first time is in the close when he realizes that Manethrall Rue's information about the location of Lord Foul's army comes five days later than he would like, a moment of despair that brings up Rue's scornful question to Elena, "Is this the man who leads your warriors?" The second time is on Kevin's Watch when he sees the size of Lord Foul's army, after already hearing the extremely demoralizing tale that Tull has brought.
And now, in this chapter, after being emotionally overwhelmed by beholding the Vortex of Trepidation, Troy's spirit hits rock bottom once again.
For one lorn moment, Troy wished High Lord Elena were with him, so that he could tell her this was not his fault. Then, abruptly, he realized that he had made another mistake. If he were killed, no one would know how to save the Warward. His final plan would die with him, and every man and woman of his army would be butchered as a result.
The realization seemed to push him over the edge. He plunged to his knees. The sirocco and the dust were strangling him.
Ruel shouted, "Warmark! Corruption attacks!"
At the word Corruption, a complete lucidity came over Troy. Fear filled all his thoughts with crystalline incisiveness. At once, he perceived that the Bloodguard was trying to undo him; Ruel's impenetrable fidelity was a deliberate assault upon his fitness for command.
The understanding made him reel, but he reacted lucidly, adroitly. He took one last look around him, saw one or two figures still surging back and forth through the livid anguish of the dust. Ruel was moving to capture him. Overhead, the dark birds dropped toward the ruins. Troy picked up a rock and climbed to his feet. When Ruel touched him, he suddenly gestured away behind the Bloodguard. Ruel turned to look. Troy hit him on the back of the skull with the rock.
It is not easy for me to forgive Troy for doing this to loyal Ruel, even when taking into account the incredible strain Troy is under. Yet he seems to recover when he is in the dire situation of being trapped by three of Foul's creatures (humans warped by the Illearth Stone, I'm guessing). He is thinking clearly enough to play dead until he can kill the last of them, and is mentally lucid enough to remember calling his Ranyhyn. And there is even an unexpected moment of humor for the reader, when one of those creatures tries to kill Troy by kicking him in the chest and breaks his own foot on Troy's breastplate. It's grim humor, granted, but humor nonetheless for me, as I usually feel pretty tense when reading this chapter. Something about that moment briefly breaks up the tension for me!