Animate Dead - per corpse or monster?

[Story]

According to the text of Animate Dead, you need a separate gem for every corpse being animated. I have a feeling this was written under the assumption that 1 corpse = 1 monster. But some monsters, such as the Necrosis Carnex require multiple corpses. Is it supposed to cost extra to animate such monsters?

[BowStreetRunner]

Necrosis Carnex would not qualify for Awaken Undead - it has an Intelligence score and undead with Intelligence scores are unaffected. Is there another undead creature with no INT that would pose the same problem of multiple corpses per monster?

[Story]

Oops, I meant to say Animate Dead.

[Phelix-Mu]

In terms of balance, necrosis carnex is more than worth a few extra onyx. A very useful tool for the necromancer, the undead general and his shambling horde, or the humble lich. Way better than normal zombies and skeletons (barring creative Corpsecrafter-tree usage, and bomb-zombies aren't normal anyway).

I think there are no monsters like necrosis carnex in the multiple corpse usage department in core. So it makes sense that the spell doesn't take this into account in it's description.

[Talderas]

According to the text of Animate Dead, you need a separate gem for every corpse being animated. I have a feeling this was written under the assumption that 1 corpse = 1 monster. But some monsters, such as the Necrosis Carnex require multiple corpses. Is it supposed to cost extra to animate such monsters?

Animate Dead's material cost is an onyx worth 25xHD gold of the animated dead. Animating a 10HD monster would require an onyx worth 250gp. Animating 10 1HD monster would require 10 onyxes worth 25gp each. The cost wouldn't change, just the number of onyxes. Can you even make a Necrosis Carnex with animate dead since the spell says it creates skeletons and zombies?

[BowStreetRunner]

The entry for Necrosis Carnex does specifically indicate the Animate Dead spell is used. Since the spell states you need a gem worth at least 25 gp per Hit Die of the undead then the 4HD Necrosis Carnex would need a gem of 100 gp. There is also an additional requirement of 10 lb of cold iron bands worth 200 gp mentioned in the Necrosis Carnex entry. I believe this total - a 100 gp black onyx and 200 gp iron bands - would be all that is required, regardless of the fact that the creature actually uses flesh from more than one corpse. (You are really only animating one corpse - it just has a few extra body parts is all!)

[The Viscount]

Use the normal costs for Animate Dead. The rules were written back when you could only make skeletons and zombies, and necrosis carnex has no special notes about onyx cost. You are only animating one being, so cost is the same.

[Ravens_cry]

One question I've always had, can spending more (or less) on gems up (or down) grade the HD of the resultant undead?

[Elderand]

One question I've always had, can spending more (or less) on gems up (or down) grade the HD of the resultant undead?

By RAW ? No, if you don't use precisely the amount of obsidian needed, it just fail. Altough I don't think it would be game breaking to allow it to make a weaker undead from a stronger corpse as a house rule. Making a stronger undead with more gems might be a problem though.

[TheifofZ]

Altough I don't think it would be game breaking to allow it to make a weaker undead from a stronger corpse as a house rule. Making a stronger undead with more gems might be a problem though.
Well, the created undead monster is only tangentially related to the corpse it's raised from, and some undead that are animated automatically have their own stats with no relation to the corpses they're created from.
Going by RAW the spell can't be cast without the appropriate amount of Onyx gems, but it might be possible to have the spell "fizzle" by creating a weaker undead than was intended to be created. (For example, creating a group of 1 HD skeletons in place of the Necrosis Carnax)

[Zomg Zombies]

As a house rule I've always played that you only use up the onyx needed; for example you got a 500gp chunk of onyx, and you come across a 2hd corpse. After animation, you're left with a 450gp onyx (either the gem shrunk by 10% or the end you touched to the corpse flakes and crumbles away, worthless).

Also the mechanic works better if you use reach spell: the spell is cast first through the onyx, then as a ray to the corpse; instead of walking to the corpse, loading up the gps of onyx into the eyes and mouth, and sneaking back to the original place so you can cast with reach.