Landwyrm, Swamp (CR 20)

Gargantuan Dragon (Aquatic)
Alignment: Usually chaotic evil
Initiative: +4; Senses: Listen +30 and Spot +37
Languages: Draconic and Common

AC: 30 (-4 size, +24 natural), touch 6, flat-footed 30
Hit Dice: 36d12+288 (522 hp)
Fort +28, Ref +20, Will +23
Speed: 30 ft., swim 20 ft.
Space: 20 ft./15 ft.
Base Attack +36; Grapple +62
Attack: Bite +46 melee
Full Attack: Bite +46 melee and 2 claws +44 melee
Damage: Bite 4d6+14/19-20, claws 2d8+7
Special Attacks/Actions: Frightful presence, hypnotizing gaze, snatch
Abilities: Str 39, Dex 10, Con 26, Int 17, Wis 17, Cha 24
Special Qualities: Amphibious, resistance to acid 30
Feats: Alertness; Blind-fight; Cleave; Combat Reflexes; Great Cleave; Improved Critical (bite); Improved Initiative; Multiattack; Persuasive; Power Attack; Snatch; Stealthy; Track
Skills: Bluff +43, Concentration +26, Diplomacy +50, Gather Information +9, Hide +12*, Intimidate +25, Jump +32, Knowledge (local) +21, Knowledge (nature) +21, Listen +30, Move Silently +36, Sense Motive +39, Spot +37, Survival +39, and Swim +14
Advancement: 37-39 HD (Gargantuan); 40-48 HD (Colossal)
Climate/Terrain: Warm marshes
Organization: Solitary pair, or cluster (3-6)
Treasure/Possessions: Standard

Source: Draconomicon

Frightful Presence (Ex): 180-ft. radius, HD 35 or fewer, Will DC 35 negates.

Hypnotizing Gaze (Su): A swamp landwyrm can hypnotize its prey just by looking into its eyes. This is similar to a gaze attack, except that the landwyrm must use a standard action, and those merely looking at the creature are not affected. Anyone the landwyrm targets must succeed on a DC 35 Will save or be hypnotized (as the hypnotism spell, except it affects a single target whose Hit Dice do not exceed the landwyrm's). If a landwyrm uses this ability in combat, the target gains a +2 bonus on the save. The effect has a range of 60 feet and lasts for 10 rounds.

Snatch (Ex): Against Medium or smaller creatures, bite for 4d6+14/round or claw for 2d8+7/round.

Amphibious (Ex): Although swamp landwyrms are aquatic, they can survive indefinitely on land.

Skills: A swamp landwyrm has a +8 racial bonus on any Swim check to perform some special action or avoid a hazard. It can always choose to take 10 on a Swim check even if distracted or endangered. It can use the run action while swimming, provided it swims in a straight line. *Swamp landwyrms have a +4 racial bonus on Hide checks when in swamp environments. This bonus on Hide check increases to +8 when the landwyrm is immobile.

Aquatic Subtype

Creatures with the aquatic subtype always have swim speeds and thus can move in water without making Swim checks. An aquatic creature can breathe underwater. It cannot also breathe air unless it has the amphibious special quality.