4 ideas
After King Arthur's forces massacre Saxon villagers, breaking a peace that Axl himself had brokered, Merlin enchants the she-dragon Querig so that her breath spreads a mist of forgetting across Britain. The aging knight Sir Gawain guards the dragon for decades to keep the mist in place. When the Saxon warrior Wistan kills Querig, the mist lifts, memories of the slaughter return, and the novel ends with the 'buried giant' of ethnic vengeance about to wake.
A peace that depends on forgetting a grievance leaves the grievance unresolved and merely suspends it. Because the injustice was never acknowledged or repaired, its return is a matter of when, not if.
Ask who administers the forgetting and who benefits from it. In the novel, the mist is imposed by the side that committed the atrocity and is maintained by its loyal knight. Official oblivion therefore shields the perpetrators' legitimacy as much as it spares the victims pain. Even Axl, a sympathetic figure, turns out to have served the regime whose crime the mist conceals.
A ferryman carries couples to an island together only if questioning shows that their love rests on a truly shared memory. Axl and Beatrice's recovered memories include her infidelity, his cruelty in barring her from their dead son's grave, and the son's estrangement. This raises the question of whether their tenderness had survived those wounds or only survived because they had forgotten them. Intimacy under forgetting can feel identical to forgiven intimacy, and only returning memory reveals which one it was.