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When an existing creature acquires a template, and that template changes the creature's hit dice, does this change the creature's existing hit points?

Example:

A 3rd level human cleric with no bonus from constitution has 8 + 2d8 hit points (full hit points at first level plus 2 more hit die). Let's say the hit die rolls were 4 and 6, so the cleric has 18 total maximum hit points. Now the cleric is infected with vampirism, which changes his type to undead and, according to the SRD, "[i]ncrease[s] all current and future Hit Dice to d12s". So, now the vampire cleric has 3d12 hit dice instead of 3d8. Is there any adjustment to his current maximum hit points of 18, which were generated from d8s instead of d12s?

I don't off-hand know if there is some acquired template that might reduce current hit dice in a manner that would make a creature's maximum hit points prior to acquiring the template become an illegal value (too high) for the templated creature. If there is a case in which acquiring a template would downgrade the hit die, then it certainly would open up this situation. Wouldn't maximum hit points need to be rerolled or adjusted in some way?

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As a house rule (hence comment and not full answer) my groups do a roll at the new die type and take the higher of the two (as long as it doesn't go over the max if the hit die is lowered) –  CatLord Mar 29 '13 at 16:28
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3 Answers 3

up vote 8 down vote accepted

Generally, template describes the situation if there will be any Hit Dice size change . Wizards online Types and Templates defines this as follows (along with changeing Constitution score)

Before applying any Constitution changes, check the Hit Dice and Hit Points entry in the template. Some templates change the number or size of the creature's Hit Dice or both. Some templates change previously acquired Hit Dice and continue to change any additional Hit Dice the creature gains. Most templates, however, change only the creature's racial Hit Dice (that is, the Hit Dice it has before adding any class levels). Most templates are fairly explicit about what happens to the creature's Hit Dice, so just follow the instructions in the template.

When you know the size and number of the creature's Hit Dice, recalculate the creature's hit points using the modifier from the creature's new Constitution modifier for each Hit Die (whatever its size).

In your example, you re-roll hit dice with new hit dice size.

UPDATE:

From D&D 3.5 Monster Manual Hit Dice entry description:

This line gives the creature’s number and type of Hit Dice (the die rolled to generate hit points), and lists any bonus hit points. A parenthetical note gives the average hit points for a creature of the indicated number of Hit Dice.

D&D generally defines base HD and additional HP (CON bonus and other bonuses if available) for creatures, but also offers a pre-calculated value for HP of that creature. So you can either calculate it or use pre-calculated value.

That means, game mechanics leave the final decision to DM (in most cases), and it is up to him. (Such as; in my games, I generally offer players 3/4 of the die they roll for their HP when they level up, and calculate NPC HDs in a similar way.)

Better you discuss that with your DM and all of your players and find a way that suits everybody.

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Does "recalculate" necessarily mean reroll? @KRyan pointed out an alternative method. Of the answers so far, this one presents the closest thing to a rule that says what to do with hit points, but it doesn't seem to go far enough on how to actually perform this "recalculation". I'm beginning to think that the DM's discretion is the official fallback. –  Matt Hamsmith Mar 29 '13 at 15:00
    
Answer updated. –  FallenAngel Mar 29 '13 at 15:50
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Most templates say specifically what you’re supposed to do. Vampire says you change the HD into d12s, so that is what you do. You can either add +2 to each previously-rolled HD (difference, on average, between a d8 and a d12), or you can just reroll. Other templates say not to recalculate these things.

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I favor the +2 approach. If you were tough before you're still tough. –  Loren Pechtel Mar 29 '13 at 2:31
    
I understand that the hit die size is changed. That doesn't necessarily mean that hit points are changed at all. You could have 18 hit points from 3d12 or 18 hit points from 3d8. @FallenAngel did find rule text that mentions recalculating hit points, but silence (so far) on exactly how to do that. I'm more curious if there is an official word on how to recalculate hit points, but your two suggested methods are appreciated and well noted. –  Matt Hamsmith Mar 29 '13 at 15:04
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You could also multiply by 6.5/4.5 (expected values from d12 and d8) and round, so 18 HP would go to 26 HP. –  Russell Borogove Mar 31 '13 at 2:24
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From the SRD, specifically Improving Monsters, in the section "Reading a Template" (emphasis added):

Most templates do not change the number of Hit Dice a monster has, but some do. Some templates change the size of a creature’s Hit Dice (usually by changing the creature type). A few templates change previously acquired Hit Dice, and continue to change Hit Dice gained with class levels, but most templates that change Hit Dice change only the creature’s original HD and leave class Hit Dice unchanged.

If the Hit Dice entry in a template description is missing, Hit Dice and hit points do not change unless the creature’s Constitution modifier changes.

Then glancing at the Vampire template (emphasis added):

Increase all current and future Hit Dice to d12s.

I think that means you reroll any hit dice that changed when the template is acquired, but otherwise leave it alone. It's true that it doesn't say what to do specifically (it says to change current hit dice but not to reroll hit points explicitly), but I can't imagine it means anything else. Moreover in the rare case where some but not all hit dice are altered by a template (ie racial hit dice but not class hit dice) I think you have to reroll the entire collection since some information is lost at that point (namely, what fraction of a character's hit points came from which pool of hit dice).

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