Kevin thought it would destroy Foul however the Power that kept him alive had stood since the creation of Time and so could not destroy him.
Both he and Foul uttered the ritual together as it had been Foul's intention all along to make Kevin carry out this deed and hurt the Land. It struck at the land and reduced everything to dust bringing about the end of the age of the Old Lords. The Staff of Law was lost during the ritual. Only a few people and works of the land survived with people fleeing to the Northron Climbs and the Wastes. The Giants survived the ritual.
However Foul was weakened for a time by it and he spent a thousand years regaining his power.