During the Stenax Massacres of 4 ABY, the Stenax species invaded the planet Tharkos, wresting control of the world from the Galactic Empire as they did to many worlds throughout the Gordian Reach.
The battle
It was an attack made by the Stenax. The Stenax had wiped out Imperial installations on Stenos and seized the Imperials' military technology, which they used to attack many Gordian Reach worlds, including Tharkos. The New Republic was able to turn the tide against the Stenax onslaught, after an intense battle near the planet.
Sources
"The University of Sanbra Guide to Intelligent Life: The Marvel Series" — Star Wars Gamer 1 (First mentioned)- The Essential Atlas
Notes and references
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