The Hand of Oberon by Roger Zelazny
With Corwin and Ganelon and Random having found the true Pattern, and seeing how the damage was done, Ganelon grabs it despite the danger. They find it's a Trump with a knife stuck through it -- not made by Dworkin, who made the Trumps they know -- and they know that it was blood that did the damage. Leading to the conclusion that Random's son Martin was probably murdered there.
Followed by a summary of the first three books that would have been better in a "what has gone before" section -- eminently skippable -- and Random, after learning something about Martin from their brother Benedict, hares after him.
Corwin goes back to the cell where he was imprisoned in the first book, and the drawing that let him escape. And the tale unfolds. As brothers and sisters are forced to reveal their plots, and who is lying becomes clear; Corwin visits his Earth home; a madman takes Corwin for his father; a warrior spares Corwin on learning his name, and more
The Courts of Chaos by Roger Zelazny
This opens with Random demanding that Corwin open the door -- he brought food -- because Corwin is reacting badly to the revelation at the end of Hand, namely that [Spoiler (click to open)]Ganelon was really Oberon in disguise.
More revelations ensue. Who is designated Oberon's heir. What Dara was up to, and who was the warrior who spared Corwin in the Courts of Chaos. What they are going to do about the black road. And the grand conclusion to this series.