The Necromancer robe bottom is an upgraded version of a Dagon'hai robe bottom, 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 robe bottom, requiring level 85 Crafting.
The Necromancer robe bottom can be reverted at any time by choosing the right-click option "Remove-kit Necromancer robe bottom", which will return a Necromancer kit and a Dagon'hai robe bottom. However, the 10 Fine cloth used to make this item will be lost.
When using the necromancer kit on the Dagon'hai robe bottoms, the chatbox will say they've "taken on a more sinister look".
The update history project is a work-in-progress – not all updates to this topic may be covered below. See here for how to help out!