Guthrum the Elder († c. 890), also known by his Christian name Æthelstan, was a 9th century Viking king of Danelaw. The adjective "ancient" comes from the fact that there is a later Guthrum, also king of the Vikings.
In 854, he killed the titular Viking ruler and embarked on a vast war against Wessex and its various successive kings, in particular Alfred the Great. In 878, when he was about to subjugate all of Wessex, he suffered a major defeat at the Battle of Edington. Consequently, he must sign the Treaty of Wedmore by which he agrees to convert to Christianity and must recognize the partition of England into two kingdoms, Wessex and Danelaw.