The Necromancer hood is an upgraded version of a Dagon'hai hat with an increased prayer bonus and a different appearance. It is made by using 10 fine cloth and a Necromancer kit on a Dagon'hai hat, requiring level 85 Crafting. This will yield a message "You use the kit on the robes and they take on a more sinister appearance".
The Necromancer hood can be reverted at any time by choosing the right-click option, "Remove-kit Necromancer hood", which will return a Necromancer kit and a Dagon'hai hat, but the 10 fine cloth used to make this item will be lost.