In D&D's Ravenloft, Akikage, also so-called shadow ninja, are the ghosts of ninja and assassins who perished before completing a mission during their lifetime. Their commitment to completing their task has carried them over into the unlife. So consumed with are they with this that they interact with the living only when necessary and never choose to communicate. Once they complete this mission, they disappear forever.
Akikage are native to Rokushima Táiyoo, but will trail their quarry to anywhere on the Demiplane of Dread. It is unknown if they are able to pursue their quarry outside of Ravenloft. Evil clerics have found a way to exploit the Akikage's strict devotion, subverting it to their own ends and enslaving the spirit. When this occurs, the Akikage becomes an Ansasshia.