A gaseous, intelligent creature that drained red blood cells from its victims and once nearly destroyed the crew of the USS Farragut.
The dikironium cloud creature was a vapor-like being that could change its density, move through space and pass through solid barriers. It attacked by draining the red blood cells of its victims, and it smelled sweetly of honey when near.
Years before Obsession, it killed much of the USS Farragut's crew, including Captain Garrovick, while a young Kirk served aboard and blamed himself for hesitating to fire. Encountering it again, Kirk pursued it obsessively and learned it was heading to Tycho IV to reproduce. He and Ensign Garrovick lured it with their blood and destroyed it with an antimatter blast.