Donald II (a.k.a. Domnall mac Causantín and Domnall II) lived from 862 to 900 and was King of Alba from 889 to 900. The wider picture in Scotland at the time is set out in our Historical Timeline.
Donald II was the son of King Constantine I of the Picts and Scots, and it is arguable that he should have succeeded to the throne on the death of his uncle, King Aedh. However, Aedh's murder in 878 was followed by the joint rule of Eochaid and Giric. In 889 Eochaid tried to gain sole control of the crown by commissioning Donald to kill Giric. This Donald did, at Dundurn near St Fillans at the eastern end of Loch Earn. He then went on to exile Eochaid, before taking the crown for himself.
However dubious Eochaid's claims to the Crown of the Picts and Scots had been, he claim to be King of Strathclyde was much stronger. When Eochaid was exiled by Donald II to Gwynedd in Wales most of the nobility of Strathclyde left with him, and Donald II combined the Crowns of the Picts and Scots and the Crown of Strathclyde, becoming the first person to be referred to in his own time as King of Alba. Alba had been an entity since the merging of the Crowns of the Picts and the Scots by Kenneth I in 843; but until Donald, Kings had taken the title King of Picts or King of the Picts and Scots.
Donald II's reign was a turbulent one. During it, much of northern Scotland fell under the control of the Vikings under Sigurd the Mighty. He was also the first Scottish King (though not the last) to be recorded as fighting against Highlanders.
Donald was killed in battle by the Danes at Dunnottar, in 900 and was buried in the graveyard at Saint Oran's Chapel on the Isle of Iona. His successor was his cousin, King Constantine II. Donald's son, Malcolm, later became King Malcolm I.