Ildulb mac Causantín, anglicised as Indulf, nicknamed An Ionsaighthigh, "the Aggressor" was king of Scots from 954. He was the son of Constantine II (Causantín mac Áeda); his mother may have been a daughter of Earl Eadulf I of Bernicia, who was an exile in Scotland.
John of Fordun and others supposed that Indulf had been king of Strathclyde in the reign of his predecessor, based on their understanding that the kingdom of Strathclyde had become a part of the kingdom of Alba in the 940s. This, however, is no longer accepted.
The Chronicle of the Kings of Alba says: "In his time oppidum Eden", usually identified as Edinburgh, "was evacuated, and abandoned to the Scots until the present day." This has been read as indicating that Lothian or some large part of it, fell to Indulf at this time. However, the conquest of Lothian is likely to have been a process rather than a single event, and the frontier between the lands of the kings of Alba and Bernicia may have lain south and east of Edinburgh many years before Indulf's reign.
Indulf's death is reported by the Chronicon Scotorum in 962, the Chronicle of the Kings of Alba adding that he was killed fighting Vikings near Cullen, at the Battle of Bauds. The Prophecy of Berchán, however, claims that he died "in the house of the same holy apostle, where his father [died]", that is at the céli dé monastery of St Andrews. He was buried on Iona.
Indulf was succeeded by Dub (Dub mac Maíl Coluim), son of his predecessor. His sons Cuilén and Amlaíb were later kings. Eochaid, a third son, was killed with Cuilén by the men of Strathclyde in 971.