Special Trait Name. Enter the description for your special trait.
Damage Resistances cold, lightning; bludgeoning, piercing, and slashing from nonmagical attacks
Damage Immunities fire, poison, Necrotic
Condition Immunities poisoned
Senses truesight 120 ft., passive Perception 25
Languages – Abyssal
Action Name. Enter the description for your action.
Multiattack. The hydra makes as many bite attacks as it has heads.
Bite. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 13 (2d10 + 6) piercing damage, plus 7 (2d6) Necrotic damage.
As a Bonus Action 1 of the heads can Spit Poison
Poison Spit: As a bonus action, 1 of the 10 heads for turn, can Spit Poison, 2d8 poison damage.
Reactive Heads. For each head the hydra head has beyond one, it gets an extra reaction that can be used only for opportunity attacks.
Legendary Action Name (Bonus Action). Enter the description for your legendary action.
Multiple Heads. The hydra has ten heads. While it has more than one head, the hydra has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.
Whenever the hydra takes 30 or more damage in a single turn, one of its heads dies. If all its heads die, the hydra dies.
At the end of its turn, it grows two heads for each of its heads that died since its last turn, unless it has taken radiant damage since its last turn. The hydra regains 10 points for each head regrown in this way.
Reactive Heads. For each head the hydra head has beyond one, it gets an extra reaction that can be used only for opportunity attacks.
Description
The hydra is a reptilian horror with a crocodilian body and multiple heads on long, serpentine necks. Although its heads can be severed, the hydra magically regrows them in short order. A typical specimen has five heads.
At the dawn of time, Tiamat, the Queen of Evil Dragons, slew a rival dragon god named Lernaea and cast her blood across the multiverse. Each drop that fell upon a world spawned a multi-headed hydra consumed by a hunger as great as the fallen god’s hatred. Great champions are known to test their mettle against these fearsome creatures.
Everlasting Hunger. A rapacious and gluttonous monster, a hydra snatches and tears apart its prey in a frenzy of feeding. When a hydra has cleared a territory of food and driven off any creatures smart enough to avoid it, it moves on to seek its meals elsewhere. A hydra’s hunger is so great that if it can’t feed, it might turn against itself, its heads attacking each other as the creature eats itself alive.
The person who made this seems to not know how to structure D&D stat blocks on D&D Beyond, which I guess is fair.