The Sack of Aporcrosan occurred on 28 March 868 when a warband of 65 Danish Vikings under Bjorn Tall-Trunks attacked the Pictish monastery at Aporcrosan on the orders of Sitriuc mac Imair. The Vikings pillaged the monastery and put its Christian monks to the sword, and this particular massacre of monks led to Christian priests stirring up opposition to Bjorn in Ireland and Cornwall.
At the end of March 868, Bjorn paid around 4,000 peningas to transport his warband of Danish Vikings from Ribe to Cell Rigmonaid in Scotland, where he aimed to carry out a raid on the moanstery of Aporcrosan. Sitriuc mac Imair, one of his lords back in Norse Dublin, had encouraged him to sack the monastery and asked for a share of the spoils (1,400 peningas). Bjorn and his warband, consisting mostly of Danish freeholders (but also including Saxon cnihts), scouted out the monastery before advancing uphill to pillage it. The monks were alerted by the ringing of alarm bells, and they took up rudimentary weapons and attempted to resist. However, the Vikings hurled stones and throwing spears at them before charging in with swords and axes and slaughtering the hapless priests. The Vikings looted ale, wine, beef, a large amount of smoked fish, cheese, chicken, vegetables, and sausage from the monastery, in addition to 2,680 peningas worth of spoils. Priests who had discovered Bjorn's actions spread word to Christian communities across Britannia, and Osraige, Ailech, and Cornwall became hostile towards Bjorn due to his mistreatment of his Christian subjects. Bjorn then returned to Ireland and gave Sitriuc his share of the spoils before returning to small-scale raids on the Emerald Isle.