Penitent

With the lighthouse keeper's help, Geralt drove the mist from the isle - then dispatched the penitent from our world for good. Now ships were once again able to sail to Arinbjorn's port, and the witcher's coin pouch grew a little fuller.

I run back inside, hasp the doors, and then I hear it – someone whispering my name. Mikkjaaal, Mikkjaaal!
Mikkjal, Eldberg lighthouse keeper

It turned out the monster haunting the isle of Eldberg and its lighthouse was a rare type of specter known as a penitent. Wherever this wraith haunts immediately becomes enveloped in thick fog and darkness - and those that wander into it usually never return. In order to defeat a penitent, one must remain in constant motion in order to avoid being struck by this agile creature, which can disappear and reappear in the blink of an eye to attack from behind. Most importantly of all, however, one must discover what keeps the monster bound to its haunting grounds - and break that bond as soon as possible.