Wormwood is the name of a "star" seen in Revelation 8:10-11, which fell to the earth in the book Soul Harvest. It was a meteor said to have the consistency of rotten wood, and it dissolved as it entered into the Earth's atmosphere, making a third of its springs of water bitter and deadly to drink.
GCASA launched a nuclear-tipped rocket to intercept this threat, with great media fanfare as is befitting of such a monumental, world-saving task, since Nicolae Carpathia had interest in focusing the public and press away from Tsion Ben-Judah at the time; however, Wormwood breaks apart spontaneously before impact. It is unclear whether Wormwood broke apart from divine will, or ablation upon atmospheric entry. It is also unclear what happens to the impactor.
The ecosystemic impact of the meteor was moderate, with only "thousands" of deaths from the chemicals it released in the waters.
In Terror in the Stadium, Wormwood is described as a comet; if it had been following a cometary orbit prior to impact, it is unknown why its presence was not detected at perihelion months in advance, or why it was classified as a comet at all if no tail was observed. The pieces of it falling on the ground are described as being large enough to be seen, indicating a very shallow angle of entry and gradual disintegration over a wide area, which is consistent with the global effects of its chemical release; the primary mass would have disintegrated in the atmosphere near Nicolae Carpathia High School for this observation to have been made by Judd Thompson Jr. and friends.