based on D&D 5th Edition
Hydra
Huge  Monstrosity , Unaligned

Armor Class 15  (natural)
Hit Points 172  (15d12+75)
Speed 30 ft. , swim 30 ft.



STR DEX CON INT WIS CHA
20  (+5) 12  (+1) 20  (+5) 2  (-4) 10  (0) 7  (-2)
Senses darkvision, passive Perception 16
Languages
Challenge 8  (0 XP)
Multiple Heads. Starts with five heads. It has advantage on saving throws for blinded, charmed, deafened, frightened, stunned, and knocked unconscious. If it takes more than 25 damage in a single turn a head dies. If all heads die it dies. At the end of a turn the hydra grows two heads for each head that died since its last turn unless it took fire damge. Gains 10 hit points for each head gained..
Reactive Heads. For each head above one it gets an extra reaction for opportunity attacks. .
Actions
Multiattack. make a bite attack for each head
Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 10 (1d10 +5 ) piercing damage.