Moving along after one of these stops, Bristlefrost noticed that there wassomething strange about the way that Bramblestar was walking. Usually the ThunderClan leader passed through the forest as silent as the wind. Now he slammed his paws down, breaking twigs and crushing dead leaves.
If there is any prey out here, he’ll drive it all away, making that racket, Bristlefrost thought.
Squirrelflight  was  staring  uneasily  at  Bramblestar  as  he  blundered around a tree stump and stumbled over a projecting root.
“Bramblestar,” she began, bounding up to him, “are you—”
Bramblestar turned on her savagely. “For StarClan’s sake,” he hissed, “leave me alone! I’m fine. I don’t need—”
He broke off with a choking cry. Bristlefrost watched in horror as his legs buckled and he sank to the ground. He made one attempt to rise, his forepaws scrabbling in the snow, then flopped back into an unmoving heap.
Yowls of consternation came from the nearby cats. Bristlefrost let out a gasp of dismay and raced up to Bramblestar’s side. Squirrelflight was already checking on him.
“He’s breathing,” she meowed, her green eyes wide with fear. “But I can’t wake him.”

Lost Stars, Chapter 16