This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.

Perhaps the most famous raid by Oeselian pirates occurred in 1187, with the attack on the Swedish town of Sigtuna by Finnic raiders from Couronia and Oesel. Among the casualties of this raid was the Swedish archbishop Johannes. The city remained occupied for some time, contributing to its decline as a center of commerce in the 13th century and the rise of Uppsala, Visby, Kalmar and Stockholm. The Livonian Chronicle describes the Oeselians as using two kinds of ships, the piratica and the liburna. The former was a warship, the latter mainly a merchant ship. A piratica could carry approximately 30 men and had a high prow shaped like a dragon or a snakehead and a rectangular sail. Viking-age treasures from Estonia mostly contain silver coins and bars. Saaremaa has the richest finds of Viking treasures after Gotland in Sweden. This strongly suggests that Estonia was an important transit country during the Viking era.
What year did the Oeselian pirates carry out a famous raid?