Sea Dragon (custom monster): CR check?

[Staminator]

I put together the below monster as an aquatic conversion of the Wyvern.  Along the way it dropped the stinger and picked up a few Manticore elements.

The challenge is *intended* for a CR11 party (or thereabouts) and I want to make them sweat; party composition does not include many optimized full casters. Please let me know how you think I did and/or if I should change stuff up!

Sea Dragon
Size/Type:     Huge Dragon (Amphibious)
Hit Dice:    12d12+60 (138 hp)
Initiative:    +2
Speed:    40 ft. (8 squares), Swim 60 ft.
Armor Class:    21 (-2 size, +2 Dex, +11 natural), touch 10, flat-footed 19
Base Attack/Grapple:    +12/+29
Attack:    Bite +21 (3d8+9) or Claw +21 (3d6+5) or Tail Slap +21 (2d8+5) or 6 Tail Spines +14 (2d6+5 19-20/x2)
Full Attack:    Bite +21 (3d8+9) and 2 Claws +19 (3d6+5) and Tail Slap +19 (2d8+5) or 6 Tail Spines +14 (2d6+5 19-20/x2)
Space/Reach:    15 ft./10 ft.
Special Attacks:    Improved grab, Tail Spines
Special Qualities:    Mucous, Darkvision 60 ft., immunity to sleep and paralysis, low-light vision, scent
Saves:    Fort +13, Ref +10, Will +9
Abilities:    Str 28, Dex 14, Con 20, Int 6, Wis 12, Cha 9
Skills:    Hide +9, Listen +16, Move Silently +18, Spot +16
Feats:    Power Attack (B), Dodge, Mobility, Multiattack (B), Elusive Target, Improved Bull Rush, Knockback
Environment:    Temperate ocean
Organization:    Solitary, pair, or flight (3-6)
Challenge Rating:    11 (?)
Treasure:    Standard
Alignment:    Usually neutral
Advancement:    8-10 HD (Huge); 11-21 HD (Gargantuan)
Level Adjustment:Improved Grab (Ex): A Sea Dragon who hits with its Bite attack can immediately make a Grapple check as a free action.

Mucous (Ex): Sea Dragons are coated in a thin layer of slimy mucous which protects their skin from bacteria while in the water and from drying out while on dry land. The mucous has a number of effects:
Aquatic Freedom: While swimming in water, a Sea Dragon is considered under the effect of a Freedom of Movement effect.
Difficult to Hold: A Sea Dragon gains a +4 bonus to resist grapple attempts.
Toxic Weapons: A Sea Dragon's mucous is highly toxic to the body of most sentient species. A successful attack by a Sea Dragon's natural weapons (including tail spines) deals 1 point of Con damage in addition to normal damage. If the target is immune to poison when struck, this damage is negated.

Tail Spines (Ex): A Sea Dragon has a double row of sharp spines along the rear of its body, just above the spine. With a snap of its tail, a Sea Dragon can loose a volley of six spines as a standard action (make an attack roll for each spine). This attack has a range of 180 feet with no range increment. All targets must be within 30 feet of each other. The creature can launch only twenty-four spines in any 24-hour period.

[RobbyPants]

I guess it's iffy, and depends on the circumstances.  The Con damage on the attacks is probably the biggest thing this guy's got going for him.  It makes the spike volley more dangerous.

In general, if this guy is encountered on land, or in the water when the PCs are ready (having cast FoM and Water Breathing, and appropriate spells), then he shouldn't be too tough.  He'd be a bit tougher in the water due to his increased mobility.  If the party encounters him in the water before casting the appropriate spells, he'd be a lot tougher.  He'd be able to grapple anyone who's not specialized in grappling and be able to drag them underwater and inflict Con damage on them each round while preventing most spellcasting.  His grapple mod is high, but not out of reach for an 11th level PC that specialized in grappling.

All in all, I think it depends on how the encounter starts.  This guy looks more like a closet troll, that will pose the biggest threat by jumping out of the water from hiding and ambushing the group.  If he's encountered on open land, he'll probably be pretty easy for the casters.  He's still enough of a bruiser to give 11th level meleers a run for their money if he can get off a full attack.  It looks like a full attack does 67.5 damage on average (if all attacks hit) plus 4 Con damage.  The party's best bet would probably be to have the meleers keep moving to prevent from being full attacked while the casters fly around and attempt to lock it down.

[Staminator]

Noted! So if the party ends up caster-heavy, try and wait till they're near the water to hit them.

Thanks

[RobbyPants]

I guess it depends on if you want to kill them or not.

If the caster has FoM prepared, then at least the beast can't grapple-drag them into the water.  If not, then they probably have one round to Dimension Door out of there.  If they get pulled underwater, they'll likely die, unless they have a Silent, Stilled save-my-ass spell prepared.  When grappled, you can only cast verbal spells, and when underwater, I imagine you can't cast verbal spells.  So the spell would literally have to have no components.

If they're level 11, they can pull a lot of these tricks off, but they have to be prepared.  I don't know how well prepared they are.  It's a judgment call on your part.

[veekie]

Well, you could also use it with murky water(seawater is hard to see very far in, granting concealment beyond a certain distance), and disappear into the murk after striking. While it's not hidden by most standards due to it's size, it does make it difficult to attack without readying actions.
This works very well if the players are on a ship, which is hard to miss by the beastie due to the shadow it casts in the water.

[Staminator]

And, after discussion with my co-DM, I now have to also create statblocks for this thing's kids. Small-sized, or thereabouts...so they can haul them back and I can send the thing's mate after it!