MM page 327.
Giant Poisonous Snake CR: 1/4 (50 XP)
Medium beast, unaligned
Armor Class: 14
Hit Points: 11 (2d8 + 2) 2d8+2
Speed:
30 ft
, swim: 30 ft
, climb: 0 ft
Skills: Proficiency Bonus +2.
Senses: Blindsight 10 ft., Passive Perception 12.
Languages: --
Challenge Rating: 1/4 (50 XP)
Actions
Bite. Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 6 (1d4 + 4) 1d4+4 piercing damage, and the target must make a DC 11 Constitution saving throw, taking 10 (3d6) 3d6 poison damage on a failed save, or half as much damage on a successful one.