From Halopedia, the Halo wiki
Ostanalus gas is a powerful necrotoxic contact poison. In most species it typically causes fatal tissue decay. Kig-Yar are not affected by its release in the air, but consuming those that have died from exposure to it is still avoided. There is no known counteragent for ostanalus.
History of use
On December 13, 2553, the Kig-Yar of Rach clan aboard the Mudoat starsloop Stolen Faith attempted to kill the Ferret team led by Veta Lopis with ostanalus. The casualness with which the crew treated the whole affair suggested to Veta that they had done this before. Unbeknownst to the crew, the humans had seen the betrayal coming, and the canisters containing the gas were switched out for harmless substitutes ahead of time. Later that day, the Ferrets used the gas against members of the Keepers of the One Freedom that boarded the ship.
List of appearances