ForceUser said:
.... the little hydra has 5 heads and is CR 4, meaning it is an appropriate challenge for 4 4th-level characters. If all five hydra heads could attack as part of one AoO, they would inflict 5d10+15 damage, or on average, 40 points of damage. And they can do this twice per round. No 4th-level character can reasonably withstand that kind of damage,
I just want to add that I can see the rule (as written in the MM, and I guess the SRD, "
Feats: A hydra's Combat Reflexes feat allows it to use all of its heads for attacks of opportunity.") Could be interpreted Either way.
Either it is able to make up to 2 AoO (in a round) with All its heads making the attacks.
Or
It is able to make a number of AoO equal to the number of heads it has, but only make the one attack roll and doing damage for that one head following that part of the rule as 'normal.'
Either way is an exception to the normal rule.
Either one of them makes sense if you are willing to see both sides.
Which one is right? I don't know.
However, I am inclined to agree with ForceUser.
And whether it is the correct interpretation of the rule or not (thereby making it a house rule) that is how I will be doing it.
Personally I feel this method fits the creature (well at least My image of the creature

) and I agree it makes it a suitable challenge for its CR this way.
I can see that the rule very well may mean "2 AoOs with all the heads making the AoO." But for me that doesn't "feel" like it is right.
Really at this point, until there is an official clarification, it depends on what part of the rule exception you emphasize when you read the rule.
Having a huge nasty AoO makes it a killer monster, having a bunch of AoOs makes it an interesting monster (IMHO of course!

)
So I guess as a house rule this is how it will be for me, and I don't see either way as wrong.
Both versions are right at this point I think.
(Although using both versions at the same time is very very wrong

hehe!)
So that is my .02 (and I hope it all makes sense!)
