Theragen was a nerve gas used by the Klingons as a conquest weapon during the expansion of the Klingon Empire. By the turn of the 23rd century, theragen had fallen out of favour, along with other chemical and biological weapons, as it was considered dishonorable by most Klingons. In 2207 it was banned, its use being strictly against imperial policy. However, in the 2250s, certain councillors pushed for the use of unconventional weapons against the United Federation of Planets. They backed the use of theragen within the empire to quell uprisings on subject planets. The councillors hoped that its effectiveness could be used to gather support for such weapons from the rest of the Klingon High Council. In 2259, one of the tests backfired, killing three-quarters of the crew of the IKS Qul qemwI