That means they don't become any more

Now, say Dampening is at 10%. Your 100k heal would then land for 90k. Mortal Strike would only affect the remaining 90k healing, putting the final amount healed at 67.5k.

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 The heal was still 25% less effective than it would have been if Mortal Strike wasn't active, but was only reduced by 22.5K -- which, you may notice, is 10% less than the previous example,

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 keeping it in line with the effects of Dampening.