he doesn't trust thirteen and expects her trials to make him confront his darkest secrets without any regard to how painful it is to him. So his personalized trial would be a gallery of moving paintings depicting his secrets, while he's chained to a wheel of death (which is tied to one of his worst memories)
the way to pass the trial is for Altair to not see himself as the monster he thinks he is so every time he accepts someone's understanding, a chain breaks