First published in 1948-1950, this was the 150th album in the long-running Suske en Wiske series, which is still going strong. It is justifiably regarded as one of the classics, the first of the Blue Albums printed in Kuifje, the Dutch language version of Hergé's Tintin magazine. Suske, Wiske and their friend Lambik investigate ghostly goings-on in the museum where Brueghel's Peasant Wedding is on display, and are approached by the shade of a Spanish nobleman who is doomed to wander the earth because he failed to deliver a royal command to prevent the Duke of Alba from bombarding the (fictional) town of Kriekenbeek during the Dutch Revolt. Suske, Wiske and Lambik are transported by the ghost's magic back to 1565 where they have numerous adventures with Brueghel, the Duke of Alba, and other historical and fictional personalities, including a thrilling chase round the battlements of Brussels town hall.
Wiske: "Lambik! Sh! I heard something!"
Lambik: "Probably the guard. You know very well that I don't believe in ghosts."
It's a little anachronistic, as the Duke of Alba did not come to the Low Countries until 1567. Also one has to wonder why the ghost himself, who has a comedy gold Spanish accent (unlike the other Spanish characters, whose speech is reported in normal Dutch) did not take care of delivering the letter in spectral form having failed to do so when alive. There's an odd sequence where we lose the three main characters and follow the mayor of Kriekenbeek for a number of pages. But basically it's a decent example of the Flemish storyteller, picking up on the themes of Prince Valiant but making them his own.
It came to the top of my pile as the most popular non-English language comic on my unread list in LibraryThing. Next up on that list is De maagd en de neger, by Judith Vanistendael.