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When an intelligent undead creature survives for 100 years or more (or when the DM decides to create an undead monster with a twist), there is a 1% chance that its connection to the Negative Energy Plane grows more mature. When this "evolution" occurs, the undead gains this template. Each additional years 100 years of existence affords an additional 1% chance of a more mature connection, plus an additional 1% chance for each previous evolution. For example, if an undead creature's connection to the Negative Energy Plane evolved three times previously, it has a 4% chance to gain one more step of maturation the next time its age requires an evolution check.
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Bolded portions for emphasis.
As you can see, both parts use the exact same language: "an additional 1% chance". This means that both are to be treated the same and thus both cumulative. Assuming the undead has the absolute worst of luck, he still has an increasing chance every 100 years of obtaining the template. After 100 centuries, it totals up to a garuntee of 100%.
If the time existing was meant to be treated as a flat 1% chance, then it would be written more along the lines of "Each additional 100 years of existance, the undead has a 1% chance to form a more mature connection plus an additional 1% chance for each previous evolution" It would have been stated as an absolute instead of those key words "an additional".
Thus, each century of existance increases the chance of evolving, whether they have evolved before or not.
You definitely have an argument for your point. The most simple expression that conveys the same meaning wouldn't have had the "additional" qualifying adjective attached to "1% chance". The fact that the writer already used the word "additional" once in the same sentence tells me that really did mean to attach that second "additional" qualifying adjective to the phrase "1% chance". However... they could have just worded the sentence they meant to say in an awkward way.
The example they give, following immediately after the sentence in contention, makes no reference to "centuries existed". I would think that they'd put that in there.
So we both have arguments for our side... Yours is that the writers wouldn't have said additional twice on accident, mine is that they would have written the example with the "centuries existed" clause if your argument was right. So it's a judgment call.
I'll try to figure out the math for your case later on.
The fast healing bit, I'm quoting other people on. I haven't actually seen rules regarding this either.
Thank you for admitting that. I'm not sure either, and you could be still right. There may be ruling elsewhere than the core books.