You specify a point within the cast range and create a spherical yellow-green highly toxic fog with a 20-foot radius around that point, and the fog will spread around the corner. The mist will remain there for the duration of the spell, but will still be blown away by strong winds. And its existence area is regarded as heavy occlusion.

Cloudkill 5e

Level: 5th

Classes: Sorcerer, Wizard

Casting Time: 1 Action

Range: 120 feet

Components: V, S

Duration: Concentration, up to 10 minutes

When any creature enters the spell area for the first time in a round, or starts its turn in the area, it must make a Constitution saving throw. Those who fail the save will take 5d8 points of poison damage, and if the save succeeds, the damage is halved. The creature is affected by the fog effect even if it holds its breath or does not need to breathe.

The dense fog moves 10 feet away from you along the ground at the beginning of each turn. Fog is heavier than air, so it will sink to low-lying ground and even seep into ground gaps.

When using a 6th-level or higher spell slot to cast this spell, the damage of the spell increases by 1d8 for every level higher than the 5th-level spell slot you use.