n medium animal 

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

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

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

AC  19; Fort +8, Ref +11, Will +6

HP 26

Coiled 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 snake can use this reaction only if it's Coiled.

Speed 20 feet, climb 20 feet, swim 20 feet

Melee [>] fangs +11 (finesse), Damage 1d8+3 piercing plus giant viper venom

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

Giant Viper Venom (poison) Saving Throw DC 17 Fortitude; Maximum Duration 6 rounds; Stage 1 1d6 poison (1 round); Stage 2 1d6 poison and drained 1.

Bestiary, page 303.