Spoiler alert: This post is part of an in-depth discussion of Sign of the Unicorn by Roger Zelazny, which will inevitably reveal key plot points. I recommend reading the book first. It's short, I promise.
At Corwin's suggestion, all the Amberites simultaneously concentrated on contacting Brand, and succeeded in opening a clear gateway to his cell in the tower where Brand had found him. Gérard and Random freed him from his tower prison and brought him back to Amber. However, in the confusion, someone—;one of Corwin's brothers or sisters—;tried to kill Brand with a knife.
In chapter 7 the Amberites try and work out who is responsible for the stabbing, as well as the other attacks on Amber, such as the siege from the dark road and the murder of Caine, which are clearly connected.