Conflagration of Doom is a direct damage spell with a range of 30" that causes D6 Strength 4 hits. Roll a D6, on a 2+ the unit suffers an additional D6 hits and you may roll a D6 again. Every time you roll an additional D6 the requirement increases by +1, so you require to roll a 3+ the second time, a 4+ the third time and so on (a 6 always succeed). Proceed to keep rolling a D6 until you fail, and then resolve all the Hits suffered.