No, and we honestly don't know _how_ it would update.
I usually use the multiple target expressions when it comes to large multiple attack powers like these. The important things to remember with a multiple attack power is that they suffer all the same disadvantages, for example a Hydra is very boned by the blinded condition and also suffers heavily from resistances. By 18th/20th level PCs have good access to many resistances and the ability to penalize monsters attacks/damage strongly.
Having run creatures like hydras in the past their flaw is they hit very rarely and cat scratch when they do. Actually hitting like a dump truck is important and is actually a strong requirement. On the turn they have enough attacks and aren't blinded/taking a penalty or similar, they need to be able to do something significant while they can actually function.
The original Heroslayer Hydra doesn't live up to its reputation. I can tell you that. The version I used was level 25 and had the following (note this is pre-MM3, so I more or less winged this):
2d8+8 + 2d8 acid damage and +10 against a marked target. Each attack after the first that hit dealt an additional +10 as well (but I limited that to only a target that marked the hydra).
It's usual damage was 2d8+8+2d8+10+10 = 4d8+28 damage after the first attack. He had a +3 instead of +2 bonus (which actually isn't that far off what a MM3ized Heroslayer would have, given he'd be starting at level+5 for his attack, not level +3).
That definitely pounded the defender into the dirt. It also meant the hydra wasn't static, just locked down chipping away ineffectively at the defenders AC for no basically no damage at all. Be aware though that if the Heroslayer hydra isn't very good either, imagine how poorly the original MM hydras do against high paragon/epic parties.
Just making its bite do 35 damage, then letting it have 8 of them per round at +2 attack over base, at +2 attack and +5 damage against marked, with +10 damage per hit after the first... would be disingenuous.
Consider what happens when the creature has the blind condition, which is a -5 to every attack. Or has a condition that reduces damage (Like Iron to Glass). Low damage with lots of attacks is a poor strategy that never works. My heroslayer hydra challenged a party at 24th level (it was level 25). The original actual heroslayer hydra barely failed to challenge
a sixteenth level party. In fact I don't think it even bloodied anyone with eight attacks.
Damage is important and being able to lay it on even more so. If the PCs put a few conditions on it, the fight is manageable. It's when the PCs don't have conditions that the Hydra needs to pounce and show it is a threat to be taken seriously. Also, the bite attack doesn't have to target the same creature, it can target whomever it wants and ideally the monster should change targets when it downs a PC (unless exceptional circumstances exist).
If you did blindly update it to a 35 damage avg bite before its bonuses, though, you could work it up to 192 damage in the round.
It needs to be obscenely lucky to do that though and won't usually get away with it. High damage numbers sound scary in theory, but never come out during an actual game - especially when you consider the array of powers and options to 18th level parties.
Edit: And I have just had the most awesome idea for an updated heroslayer hydra now.
Edit: Holy crapsticks, I can almost do a 2for1 here with a more unified concept of the "Heroslayer Hydra" being anti-defender and the "Guardian Hydra" being a defender version for an area/item/creature of the heroslayer. I can also simplify all the word slayer in the Guardian Hydra with this new idea.
Excellent. Also yes, I know the Heroslayer Hydra is from MM2. It doesn't mean he doesn't need love.
Edit3: Yeah, the statblock is now entirely different! Much cleaner mechanics.
