This specter companions populate the dreams of unstable people... rather psychopaths. They take pleasure in pushing them to the edge...
Greifmar, according to event's storyline, is an ancient, already extinct companion who can cause nightmares and chaos. This is why Cornelia tried to resurrect it and supposedly also succeeded to do so since Erika gets it's egg at the end of the event after defeating Cornelia.
Can be obtained once all the missions were completed if the player got all three illustrations and the whole outfit.