(D&D 3.5) Campaign boss, needs criticism.

[Goonthegoof]

I'm making a monster as a kind of end boss for my campaign (an avatar of a snake god, so more the dragon than the big bad, but still one of the hardest things they're going to fight) and I figured I'd post it here and get some information on how it's likely to fare. The party in question is made of 6 tier 3 characters, but ones built by players who aren't great at optimization (crusader, bard, dread necromancer, factotum, rogue, swordsage) and I'm trying to create a monster that's fearsome but not unbeatable.

Because I'm used to building things like a player and thus tend to create creatures using existing stuff rather than make it up, I started with an ordinary 12 headed hydra, gave it the Chameleon template so it would have the Reptilian suptype, qualifying it for Ti-Khana, qualifying it for Elder Serpent, and then stuck monster of legend on top for the hell of it, which left me with this:

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Monster of Legend Elder Serpent Ti-Khana 12 headed hydra CR 20
Magical Beast 14 Warshaper 3 14d10+3d8 Space 15ft Reach 10ft Initiative +11
Huge Magical Beast (Reptilian, Augmented Animal) 20' movement, BAB +16 Grapple +41
HP 366 AC 37 (-2 size +22 natural +7 dex) (Touch 15 flat footed 30)
Str 44 Dex 24 Con 40 Int 24 Wis 24 Cha 16
Fort 30 Reflex 20 Will 15
Attack: +34 4d8+17
Full Attack: 12x +34 4d8+17
Multiattack, Improved Initiative, Combat Reflexes, Toughness
Weapon Focus (Bite), Improved Natural Attack (Bite), Quicken Breath
Snatch, Improved Snatch
Morphic Immunities, Morphic Weapons, Morphic Body, Morphic Reach
Spell Resistance 29, Detect Poison at will, Fast Healing 22
Shapechange Tiny-Large viper only,
Darkvision 60', Low-light vision, Blindsense 70', Scent
1d6str,1d6con/1d6str,1d6con poison DC 35, Greater Damage
Reflective Hide (Permanent Spell Turning)
+4 bluff, intimidate, listen, search, sense motive, spot
Command Serpents, Command 16HD or less serpents, save 14, CL 14
Mesmerizing Sway, 21 will neg, 30' radius creatures are paralyzed
for as long as effect continues +1d6 rounds
3d6/head acid breath weapon in a 15' cone, recharge 1d4, DC reflex 33

So what I want to know is how high level party of tier 3s is going to stack up against this, and whether I need to increase or decrease its power. I also need to give it another form of movement, as currently there's no way it'll catch anything. Any suggestions?

[Ryuuk]

It's initiative is high, so there's a good chance it'll get to act before all the players act. The spell turning seems to counter most of the Dread Necro's options, you've got a Crusader, Rogue and Swordsage backed up by a Bard and the players aren't optimized, so odds are one of them's going to get close to the thing to try and hurt it and take a full attack in return. The Factotum can ignore its Spell Resistance, but might fall to the turning or might join the others in trying to melee it.

As their DM, how high do you think their AC will be by the time they fight it? If it's not above 36, well, odds are the first PC to get within range will take some 300ish damage before making a ton of saves against poison.

If you're PCs realize that they need to stay away from that thing's mouths they might be looking at a long but relatively safe encounter, but anyone that's signaled out for a full attack is pretty much gone.

At what level will they be? 16? 20th? Whether its hard or not is going to depend entirely on the PCs numbers.

[Psyx]

Int 24 Wis 24
Toughness

Spot the paradox!

[UnChosenOne]

Another from of moment? You could always give it wings without any other reason that it's an avatar of god. Or just add a half-dragon (black) template (grants fly speed, acid immunity, line of acid and wings. Fly speed for a half-dragon is it's ground speedx2)