Thank you for contacting us. Radiant Curse allows you to deal your Warlock's Curse extra damage when an ally hits that creature. This can only be done once per round, This is in addition to the normal extra damage dealt once per turn by your Warlocks Curse.
To give you an example, you are playing a Warlock with Radiant Curse. On your turn, you hit a cursed enemy and deal your extra damage. On an allies turn he hits the cured enemy. This allows you to deal the extra damage from Radiant Curse. If another ally hits the cursed enemy later in the turn, you wouldn't deal the extra damage then since it is limited to once a turn.
