Limark was a Redguard knight who resided in the Shivering Isles. Limark, at some point, killed himself and his soul is cursed to walk the Hill of Suicides; the only way to be freed is if somebody finds his skull and returns it.
Limark's skull is found in the ruins of Milchar.
In the Shivering Isles, it is a crime to commit suicide; the punishment for doing so is to be trapped on the Hill of Suicides. The only way to break the curse is if somebody is to return the skull to its spirit.