Wyvern's Fire (Gunlance)

This attack takes 2 actions to use over 2 of your turns, but deals a devastating 4d12 force damage and knocks the target prone, or half as much on a successful Dex save against your spell DC, and the target is not knocked prone. Note that once used, at the start of each turn you must roll 1d6, on a result of a 6 the ability recharges, much in the same way as some monster abilities.