An alien assumed the form of Captain Kathryn Janeway's father, Vice Admiral Janeway in 2373 and tried to lure Kathryn Janeway to her own death after her shuttle, the Sacajawea, had crash landed on a planet with Chakotay.
The alien had invaded her cerebral cortex, causing hallucinations. It told her that she had died in the shuttle crash, and tried to convince her that she should accept her death. It pretended to be the spirit of her father, come to help her cross over to the afterlife. Unbeknownst to Janeway, the non-corporeal being wanted to coax her spirit away from her body in order to feed on it. Janeway, however, sensed that she was really still alive and that The Doctor was trying to save her.
The alien told her that this was a hallucination and urged her to give up on her former life. Janeway knew that her real father would have wanted her to struggle to survive, and thus realized that the apparition was not who he claimed to be. She refused to go, the alien left her cerebral cortex and she regained consciousness.
Before departing, the alien swore that Janeway would inevitably come to his matrix one day, and that then her essence would feed him for a very long time. Janeway likened the being to a vulture, while Chakotay likened him to a spider attempting to lure a fly. (VOY: "Coda")
This alien entity, portraying Janeway's father, was played by Len Cariou.
According to the Star Trek Encyclopedia, 2nd ed., p. 84, this being was described as a "consciousness parasite".