The King of the Dead at the Dark Palace Light Novel 1

When a sickly boy succumbs to illness only to reawaken as a weakly undead, his first thought is not of horror, but joy. Sadly, his delight is cut short by the realization that he is still not in control of his own body. The excruciating pain that plagued him in his previous life has simply been replaced by the necromancer that brought him into this one. Now, to win true freedom, he must put his unlife on the line and fight for his dark master...!