Ryloth, also known as Twi’lek, and Twi’lek Prime, was the harsh, rocky homeworld of the Twi’leks, an Outer Rim Territories world located on the Corellian Run and forming one endpoint of the Death Wind Corridor. One side of the planet perpetually faced its sun and the other remained in darkness, a phenomenon known as tidal locking. The dayside was referred to as the Bright Lands and the night-side was known as the Nightlands.
Ryloth, the second planet in the Ryloth system, possessed a thin, but breathable atmosphere. The planet was orbited by five moons, which had subtle tidal effects on the planet’s liquid core and underground springs. It had little or no surface water except for patches of water ice on the night side.
It was a planet of extremes: dry, rocky landscapes of deserts, mountain ranges with mist-covered peaks, shadowy valleys, alternated with snowy wastes, joined by lush temperate bands which concentrated much of its population.
Because of its unusual day-night rotation period, atmospheric conditions on Ryloth’s surface were characterized by turbulent seasons and extremely high winds (reaching 500 kilometers per hour) scouring the surface and sculpting the landscape and unpredictable bursts of heat called heat storms, which could reach 300 degrees centigrade. The more extreme heat storms sometimes moved across the terminator and reached miles underground, making caves near the surface temporarily uninhabitable, before eventually dying out on the night side.
Along the terminator lay a thin habitable border, a twilight realm between the two extremes. Along this narrow strip were the series of caverns where some of the Twi’leks decided to build their underground cities, blockaded with thick blast doors and portcullises to keep out the vicious monsters that prowled the Bright Lands, especially lyleks.