beyond the wide world's end

Found it! First paragraph of Dust 11:

The last time Tirian had been in Cair Paravel, he’d been lost in the pressing crowd of Narnians gathered in the castle courtyard to see Prince Bahadur of Calormen formally given the rule of Calormen’s latest client state. That had been in the first year of Calormene occupation and the whole of Narnia had still been in shock; no one had quite realized that Aslan’s appearance had all been a clever trick to give the Calormenes passage to a land that was otherwise nearly impenetrable. Bahadur had produced the lion’s pelt after he’d risen with the thin gold circlet of regency on his brow. Tirian can still remember the way his breath caught in his throat, sick with horror. Surely not Aslan – he’d thought. Surely even the Calormenes would not dare – But dare they had, and after that there was no faith to be found in Aslan in Narnia.

I am at this point willing to contradict myself, though. :) Or change the names or dates.