Feats: A hydra’s Combat Reflexes feat allows it to use all its heads for attacks of opportunity. (Monster Manual 156)
No further information is provided, and I'm at a loss. Here're the questions:
- Can a hydra make in a round only a number of attacks of opportunity equal to its number of heads? Equal to either the higher of its number of heads or 1 plus its Dexterity bonus? Equal to its number of heads plus 1 plus its Dexterity bonus? Something else?
- Does this natural ability grant a hydra special permission to bite a foe with up to all of its heads when a foe provokes a attack of opportunity? For example, Abe is in the threatened area of a 5-headed hydra. Abe foolishly retrieves an item from his belt pouch. This normally provokes one attack of opportunity from each creature that's threatening Abe. Can the 5-headed hydra instead make up to 5 attacks of opportunity—one for each of its heads—against Abe?
If it makes a difference—or makes answers easier to compose—, this specific 5-headed hydra has a Dexterity score of 20.
Note: When I've used hydras in the past, they've been in their homes: in marshes where the PCs could evade them or stay out of their reach. The hydra for an upcoming encounter, on the other hand, is—I'm not making this up—in a 50-ft. by 50-ft. room. (Yes, Castle Amber is a very long adventure.) This hydra's inexplicably tiny lair means I can no longer avoid this baffling bit of rules text.