He betrayed his people to Nagash, the God of the Dead and was transformed into a wraith. 
In the Time of Tribulations, he was tasked with directing vast legions of skeleton labourers to gather vitrified realmstone to construct a vast nexus of power in the form of an inverted pyramid in the Realm of Shyish
|You who think yourselves safe from the fate you so richly deserve. Look upon these legions of the dead, and know that you will soon be amongst them, with maggots in your belly and dust in your lying mouths.
~ Keldrek, Emissary of the Prime Innerlands.