Hit: "1d6 + Charisma or Constitution modifier radiant damage. The first time the target moves closer to you on its next turn, it takes an extra 1d6 + Charisma or Constitution modifier damage." Level 21: "2d6 + Charisma or Constitution modifier damage."[PH:131][U:W]
Dire radiance is an at-will power available to warlocks at 1st level. It is granted automatically to warlocks who choose the star pact option for their Eldritch Pact.
The warlock update added the option to use Charisma instead of Constitution for dire radiance's attack and damage rolls. Previously, only Constitution could be used.