Kaloyan of Bulgaria
Boniface's rule lasted less than two years before he was ambushed by Tsar Kaloyan of Bulgaria and killed on September 4, 1207. The kingdom passed to Boniface's son Demetrius, who was still a baby, so actual power was held by various minor nobles of Lombard origin. These nobles, under the regent Oberto, began plotting to place William VI of Montferrat, Boniface's elder son, on the throne, and openly defied the Latin Emperor Henry of Flanders. Henry marched against them in 1209 and forced their submission. As a result, Henry's brother Eustace then became regent for Demetrius.