unique n medium animal 

Perception +7; low-light vision, scent (imprecise) 30 feet

Skills Acrobatics +9, Athletics +8, Stealth +8, Survival +6

Str +2 Dex +4 Con +3 Int -4 Wis +1 Cha -2

Survival +6 Str +2, Dex +4, Con +3, Int –4, Wis +1, Cha –2.

AC  19; Fort +7, Ref +10, Will +7

HP 27

Attack of Opportunity [R] Trigger A creature within the monster's reach uses a manipulate action or a move action, makes a ranged attack, or leaves a square during a move action it's using. Effect As Attack of Opportunity, but the serpent can use this reaction only if it's coiled.

Speed 20 feet, climb 20 feet, swim 20 feet

Melee [>] fangs +11 (finesse), Damage 1d8+2 piercing plus 1d4 electricity

Coil [>] The serpent uses an action to coil itself; increasing its reach with its fangs from 5 feet to 10 feet. After the serpent strikes with its fangs, it becomes uncoiled.

Lightning Rattle [>>] The serpent rattles its strange metal tail and unleashes a bolt of lightning in a 60-foot line. The bolt deals 2d10 electricity damage (DC 17 basic Reflex save).

The Fall of Plaguestone, page 20.