I'm developing an encounter where the opponent is a human wizard with an undead template applied. In becoming undead, the opponent's HD change from d4 to d12 but I'm not sure how this changes the actual hp of the advanced creature.
The MM hasn't helped much. The following quotes are from the "Templates" section in the "Advancing Monsters" chapter:
Hit Dice and Hit Points: 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.
[b]
If the Hit Dice entry is a template description is missing, Hit Dice and hit points do not change unless the creature's Constitution modifier changes.When you know the new Constitution modifier and any change to Hit Dice size, recalculate the templated monster's hit points. See Hit Dice and Hit Points above.
The first passage tells you how HD size can be affected by a template, including changing the size of existing and future HD, but does not explain how to recalculate hp's when existing HD are changed.
The second passage seems to imply that changing the HD size should definitely impact the recalculation of hp's for the templated creature, and only refers back to the first passage instead of providing an explaination on how to do it.
Simply re-rolling all the hp bothers me, because no relationship is maintained between the original version's hp and the advanced version's hp.
Leaving the hp as is (adjusted for having no Constitution of course) also bothers me because, even if the base creature (in this case) had max hp, that would only be 1/3 of the max hp from the new HD - far too weak, relative to the original.
Now, as I am someone who is not afraid of math there is one method I could use that addresses both my concerns - calculate the ratio of hp to max hp for the base creature and use that to find a comparable hp value for the advanced creature:
hp_advanced = ( hp_base / hp_base_max ) * hp_advanced_max
I suspect that even this much math is too much for it to be even close to the "official" method for dealing with this issue. Hopefully someone out in Dungeon-land can help me out.
:-j
Jenni
Generally, NPCs should have an average amount of hit points. Find this number like so:
(x+y)*z
Where X is the average for the hit die:
d4 - 2.5
d6 - 3.5
d8 - 4.5
d10 - 5.5
d12 - 6.5
Where Y is the Constitution modifier, and Z is the number of hit dice the creature possesses. This becomes slightly more complicated when dealing with multiclass characters or lycanthropes, but once you've got this bit figured out, the rest should fall into place.
As you state that NPCs should have average hp regardless of HD, the base creature (Wiz 9) would start with the average of 9d4+Con and the advanced creature would end up with the average of 9d12 (Undead have no Con). This, of course, has the side effect of maintaining the ratio between hp and max hp but only works if you start and finish with exactly "average" hp.
Sometimes, though, you want an important elite NPC to have slightly better than average hp which is the situation I'm in. Which brings me back to the math solution again...
:-j
Is it really that bad to recalculate hit points by rolling the 9d12? I mean, pretty much every part of the NPC's body is altered when becoming undead. Why not the hit points?
As you state that NPCs should have average hp regardless of HD, the base creature (Wiz 9) would start with the average of 9d4+Con and the advanced creature would end up with the average of 9d12 (Undead have no Con). This, of course, has the side effect of maintaining the ratio between hp and max hp but only works if you start and finish with exactly "average" hp.
Sometimes, though, you want an important elite NPC to have slightly better than average hp which is the situation I'm in. Which brings me back to the math solution again...
:-j
Just maintain the difference in averages. So if you gave the NPC an average of 3hps (+con) per HD give the undead version 9hps per HD (no Con bonus).
Alternitivly you have increased the monsters potential hps by a factor of three. Eliminate the con bonus from the original wizard and simply multiply what you have by three.
I guess the answer to my question is "Do whatever you think makes the most sense."
For some, using the ratio of hp to HD seems preferable because it maintains a direct relationship between the base creature's hp and the advanced creature's hp (BTW Jeremy: Your suggestion is actually the same thing -- you're just describing it differently. :-D)
For others, re-rolling 9d12 seems reasonable because so much else has changed when the template was added.
For me, failing a definitive "How To" statement, the ratio option feels the most best option, so that's what I'm going to use.
Thanks to everyone for responding to my question. :-)
Jenni
:-j