Valygar's Body will be obtained if Gorion's Ward decides to kill Valygar Corthala in the quest The hunt for Valygar Corthala. It can used to open the door of the Planar Sphere. One can, however, enter it also by taking him there alive as a party member.
Location
Baldur's Gate II: Shadows of Amn
In-game description
“ | This is the body of Valygar. | ” |