Lightning Weapon+Shard of Lightning does the same thing, though it takes up your weapon enchant. Genasi+Shocking Flame+Shard of Lighting or Shard of Fire. If you are playing a class that uses Bursts/Blasts using the Shard+Radiant Weapon is really good, because otherwise it can be tricky to get an item bonus to your damage (Iron Armbands only applies to melee attacks, not bursts/blasts).
For the OP if you can get a Ring of the Radiant Storm it is fantastic combined with Painful Oath (since it adds the Radiant keyword). Take all your damage dice for an attack, roll them twice, use whichever total is higher.
