You chose a creature you can see, that creature needs to roll a saving throw, if it succeeds it is not affected by the spell.

As you see your target, you can use this spell trap it in a circle of light wrapping its body and getting tighter to do damage, forcing it in the conditions of blinded and restrained. 

At the beginning of the creature's turn, while it is under this spell it takes 1d4 radiant damage and at the end it must roll a strenght saving throw.

Outside of combat, this spell only traps the target, giving him the conditions of blinded and restrained.