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Cumulative damage is a variant form of ongoing damage. The difference arises in that with each failed save, the victim takes the current value in damage, then potential damage increases by the innitial value. This is essentially shorthand for the following:
Hit: Ongoing X damage (save ends).
First failed save: Ongoing 2X damage (save ends).
Second failed save: Ongoing 3X damage (save ends).
(and so on...)
For example, a creature taking cumulative 5 damage will receive 5 damage on the first failed save, 10 on the second, 15 on the third, and so forth - adding 5 damage each time.
For purposes of stacking, treat this exactly as though it were onging damage. Therefore, as a further example, a creature taking ongoing 10 fire damage and cumulative 5 fire damage would take the greater of the two on any given turn - 10 fire damage on the first two failed saves and the cumulative value thereafter (assuming it doesn't save successfully).