Harrow is a psychic bug-like creature, who is secretly a stowaway on Galactus's Ship. He feeds off the fear of creatures whose world's are about to be destroyed by Galactus, mentally taking control of civilians and killing other innocents from inside their bodies. Harrow first came to the attention of Nova during the evacuation of a planet called Orbucen, which was about to be consumed by Galactus. There, he was killing innocents as he usually does, but was discovered by Nova. Upon discovering him, Nova was able to trap him inside a gravimetric node.
However, despite his seeming defeat, Harrow found a way to escape the node, and terrorised Orbucen once more. Nova stayed back to stop him, and Harrow was seemingly burned by the engines of a refugee ship leaving. However, it turned out that Harrow had survived, as his true form was on Galactus's ship. When Nova travelled there, he started to fight Harrow's true form, which caught the Silver Surfer's attention. Upon Nova explaining what Harrow was, the Surfer notified Galactus, who mercilessly destroyed Harrow.
Harrow was able to psychically enter people's minds and control them. If his host body was destroyed, his astral form would simply enter another. He also had some way of manipulating the biology of those who he controlled, being able to create claws upon taking control. Harrow also had something called a "Cone of Psilence", which prevented Galactus discovering him.
Nova was immune to Harrow's powers at a distance, as he had strong psychic shields. However, once Nova show his true form, Harrow's telepathic power was too much for him.