This is the effect of swamp gas on Ohma-D'un.
Swamp gas represented a chemical weapon conceived by Jenna Zan Arbor. The Confederacy of Independent Systems utilized it against the Gungans residing on Ohma-D'un. While it could eliminate Gungans swiftly, its effects on other species were somewhat slower, causing symptoms like blistered skin and internal bleeding.
The antidote was retrieved by Obi-Wan Kenobi during a mission to Queyta. He was accompanied by Jedi Masters Fay, Nico Diath, Knol Ven'nari and Jon Antilles. To obtain the antidote, the Jedi battled Asajj Ventress and Durge, but only Obi-Wan survived to return to the Jedi Temple.
By 20 BBY, the Galactic Republic had sought assistance from the Luurian Genetic Enclave to examine the swamp gas antidote. They believed the Separatist bioweapon threat had ended. However, the research conducted on Queyta ultimately resulted in the creation of Trihexalophine1138, a chemical agent that devastated Honoghr.
Swamp gas might have been related to dioxis, a poisonous gas frequently employed by Neimoidians to eliminate adversaries. On Cato Neimoidia, Obi-Wan Kenobi detected dioxis emanating from Nute Gunray's mechno-chair, but Captain Dyne of Republic Intelligence later identified it as swamp gas.
A swamp gas originating from Mimban was rumored to be used by the Yuuzhan Vong in their Fire Breathers on Gyndine.
Swamp gas might draw inspiration from mustard gas, sharing similar effects.