Sarah Kerrigan is transformed into the Queen of Blades on Tarsonis in StarCraft II’s backstory.
During the original StarCraft, the Zerg invade Tarsonis, the capital world of the Terran Confederacy. Arcturus Mengsk uses a Psi Emitter to draw the Zerg into the Confederate capital, then abandons Kerrigan during the evacuation. The Zerg capture her, infect her, and transform her into their powerful new leader.
Tarsonis is therefore both a political center and a turning point in the series. Its fall destroys the Confederacy and allows Mengsk to establish the Terran Dominion. Kerrigan’s transformation also drives the later conflict between the Zerg, Dominion, and Protoss.
Char is often mistaken for the answer because it is strongly associated with Kerrigan’s Zerg rule and later campaigns. However, Char becomes an important Zerg world after Kerrigan’s infestation; the original transformation itself follows the battle on Tarsonis.