Krum the Horrible (Bulgarian: Крум) was Khan of Bulgaria, from after 796, but before 803, to 814 AD. During his reign the Bulgarian territory doubled in size, spreading from the middle Danube to the Dnieper and from Odrin to the Tatra Mountains. His able and energetic rule brought law and order to Bulgaria and developed the rudiments of state organization.

In c. 805, Krum took advantage of the defeat of the Avar Khaganate to destroy the remainder of the Avars and to expand his authority across the Carpathians over Transylvania and along the Danube into eastern Pannonia. This resulted in the establishment of a common border between the Frankish Empire and Bulgaria, which would have important repercussions for the policy of Krum's successors.

Krum engaged in a policy of territorial expansion. In 807 the Bulgarian forces defeated the Byzantine army in the Struma valley. In 809 Krum besieged and forced the surrender of Serdica (Sofia). This victory provoked the reaction of the Byzantine Emperor Nikephoros I, who proceeded to settle Anatolian populations along the frontier to protect it. He also attempted to retake and refortify Serdica, although this enterprise ultimately failed.