Ok, I'm trying to create a semi-demonic recon killer. What I did was take a base doppleganger and added 2 racial HD so that it would be able to use the change self(polymorph) instead of change self(alter self). I then added the fiendish template and lastly I added 5 levels of rogue to the monster. However, since I wanted the monster to have a little less than normal hit points to offset the fact that it's a surprise killer when it needs to be, ie reducing its Con from 12 to 10 when adding the ability adjustments for gaining class levels the monster falls horribly short of the suggested hit points for a CR 10 monster. Table 1-1 says a CR 10 monster should have around 130 hp, but my monster only ends up with about 55 hp which is way lower than it should be even with a minor downward adjust of its Con. Can someone help me and show me where I went wrong with the hp?
Ok, I'm trying to create a semi-demonic recon killer. What I did was take a base doppleganger and ... Can someone help me and show me where I went wrong with the hp?
I don't think you did anything wrong, and the hp for 10th level creatures seems to be more of a guideline than a hard rule.
A simplified calculation for its hit dice would be:
4 hd - base doppleganger
+2 hd - additional hd you gave it
+0 hd - fiendish template gives no extra hd
+5 hd - 5 levels of rogue
11hd total. Average hp would be approx. 55hp (5 out of 8 possible hp per level, 5 times 11 = 55 hp). No con bonus since you reduced it so no extra hit points there. If you were to give it max hp (8 times 11) then you end up with 88hp.
If you think it's hit points are too low, then just give it back its original doppleganger Con of 12 and add 12 more hit points. Or bump the Con up higher.
Also keep in mind that Challenge Ratings (CR) mean that a CR 10 monster wuold be a challenge for a group of FOUR 10th-level character, if I'm not mistaken.
So a CR 10 creature might not be the same as a 10 hit dice creature. In fact, in order for a 10 hit dice creature to be any sort of challenge for four 10th-level characters, it would need a lot of special abilities and defenses if it only had typical hit points for a 10 hit dice creature.
Typically, a CR 10 creature will have several more hit die than a 10 hit dice creature, anyway.