Ychna were enormous creatures, resembling space slugs and bioengineered, with mouths exceeding eighty meters in width. Capable of encircling an orbital city, these beasts were deployed by the sentient Yuuzhan Vong species during the Yuuzhan Vong War, primarily to assault orbital structures and space stations. During the 26 ABY conflict above the planet Duro, one ychna was deployed, consuming the city of Orr-Om in orbit. In the war's final year, another ychna participated in an attack against Caluula Station.
The Yuuzhan Vong species, who are sentient, bioengineered the massive ychna, creating creatures that bore a resemblance to space slugs. These ychna had a wedge-shaped head, and a mouth that spanned over eighty meters. Additionally, they possessed a pouch above their blowhole, positioned on their dorsal side, which Jedi Master Mara Jade Skywalker believed functioned as a life support system. The ychna's mouth was immensely powerful, capable of easily crushing the metal used in the construction of space stations. Their thick hide enabled them to endure the vacuum of space, and they could drain all the energy produced by a space station's deflector shield generator. In the vacuum of space, ychna sometimes required the assistance of gravity-manipulating dovin basals to be towed into position. Despite their enormous proportions, a proton torpedo launched into its mouth could kill a ychna.
Ychna were utilized to assault space stations and other orbital platforms, consuming their mechanical targets voraciously until they became bloated; their hunger was nearly insatiable. They would sometimes tear off pieces of a station, consuming the separated fragments of the superstructure. Occasionally, ychna would target nearby enemy starships. When attacking space stations, they often wrapped themselves around the orbital platforms to crush them.
Ychna were bioengineered around 25 ABY, and subsequently deployed by the Yuuzhan Vong during their invasion of the galaxy controlled by the New Republic. The Yuuzhan Vong primarily employed ychna to destroy the New Republic's mechanical space stations. During the Yuuzhan Vong attack on the planet Duro in 26 ABY, Warmaster Tsavong Lah utilized a ychna to obliterate the orbital city of Orr-Om. Lah also threatened the defenders of Duro, stating that the ychna would destroy them if the Yuuzhan Vong forces were attacked. However, Jedi Anakin Solo managed to destroy the ychna by launching a proton torpedo down its throat using his X-wing starfighter, though the Yuuzhan Vong still conquered the planet.
During the war's final year, 29 ABY, a ychna participated in an attack against Caluula Station, orbiting the planet Caluula. The ychna disabled the station's deflector shields before tearing a hole in the station's exterior, which Yuuzhan Vong warriors used to board and seize control of the facility.
The ychna made their debut in The New Jedi Order: Balance Point, a 2000 novel from The New Jedi Order series, authored by Kathy Tyers. They also appeared later in James Luceno's 2003 novel, The New Jedi Order: The Unifying Force, which served as the series' concluding book. It is worth noting that within The Unifying Force, "ychna" is twice misspelled as "yncha."