St. Brice's Day Massacre — Anglo-Saxons Kill Danish Settlers
English King Æthelred the Unready ordered the massacre of all Danes living in England, fearing they would support a Viking invasion. Among the dead was Gunhilde, sister of Danish King Sweyn Forkbeard, who was reportedly tortured to death.
The massacre enraged Sweyn Forkbeard, who launched devastating retaliatory raids on England and eventually conquered the entire country by 1013.
Outcome
Massacres trigger Sweyn's invasion; Danes conquer England within 11 years