The Sundering: Rise of the Witch King is a mod for Medieval 2: Kingdoms and based around the fantasy world of Game workshop's Warhammer universe during the civil war between the Elves, the mod's campaign opens with the Death of Bel Shanaar in -2751 IC.
You do not need this if you have already downloaded the parts separately.
1)Download all parts to the same folder
2)Run the TheSundering.exe and let the installer do the job
3)Make sure the path is correct (C:\Program Files (x86)\SEGA\Medieval II Total War\mods\The_Sundering) or wherever you have installed your game.
4)If you are using Steam be sure to follow this tutorial to get the mod to work.