Sedenach is a planet, home to the Bismuth Mountains.
Kara Zor-El and her parents would spend their solstices on Sedenach. At one point, Kara visited the Bismuth Mountains and got to see their crystal staircases.
Mon-El once took a girl to Sedenach, presumably for romantic reasons.
Thomas Coville somehow found a sample of Sedenach dust on Earth, which was used by his acolytes to make a Worldkiller.