The backstory behind King Caspian's loss of his wife (killed by the Green Lady in snake form) and son (abducted, kept as a hostage andbrainwashed for years). Especially when Lord Drinian wants to atone for his mistakes that led to this to happen by allowing the king to execute him with his own hands, but Caspian cries and hugs him, saying, "I have lost my queen and my son: shall I lose my friend also?"
Prince Rilian's ranting as he's tied to the chair. It starts with him sadly remembering his life in Narnia, only to notice Jill, Eustace, and Puddleglum and beg them to free him. When they hesitate, he becomes so desperate that he begins shouting increasingly violent threats at them, to the point where he's foaming at the mouth when they do free him. Keep in mind that this has been going on for years, him "waking up" nightly, only to find he can't escape. Consider also that after he's freed, Rilian is generally kind and reasonable. You have to wonder how horrific things were for him to be so desperate to be freed.
The gnomes. They had been magically summoned, against their will, from their home in Bism by the Lady of the Green Kirtle, who had made them unable to remember who they were or where they belonged. In the words of Golg, when he's been released from the spell: "We couldn't do anything, or think anything, except what she put into our heads. And it was glum and gloomy things she put there all those years. I've nearly forgotten how to make a joke or dance a jig." Thankfully, the killing of the Lady breaks the spell, and an underground chasm opens, allowing them to get home.
The ending par excellence. Caspian, who readers last saw as a vital, energetic young man deeply in love, dies in his son's arms, just after reuniting with him when he had lost all hope of his return. All the characters we knew and fell in love with, like Trumpkin, are ancient and decrepit and clearly do not recognise the clearly upset Eustace. Thankfully, Caspian is later rejuvenated and comes to "our world" to act as a Bully Hunter to Jill's bullies.
When arriving in Narnia, Eustace was unable to recognize Caspian because he was so old and changed. Learning that sickly, grieving old man is actually the friend he shared such wonderful adventures with clearly dismays Eustace.
Caspian's death is also especially cruel for Rilian. The prince has been brainwashed by the Emerald Witch for a very, VERY long time, and while his freedom is a happy thing, it comes at the price of him coming home only long enough to see his father for a final few moments before Caspian dies.
