Hydra and earthworm had one thing in common. You cut one head of hydra, then you get two heads.
Hercules along with his cousin, won hydra in his battle by chopping the head and burning the torso with sword dipped in hydra's poisionous blood.
So, battle with him was almost impossible. He had too many duplicates of him. So, there came Kali- scary avatar of Goddess shakti consort of alpha god Shiva, and with her huge tongue sucked all the blood shed by the daemon and chopped him and killed him.
Kali (Goddess of destruction) - This is an interesting avatar of goddess Durga.
And one more interesting thing to note, after killing Raktavijia she danced happily coupled with rage in the graveyard and on her wounded hubby Shiva who was lying unconsicous among the corpse and she danced on top of him as well, which she didnt realise for very long time and after she realised she came back to her normal form :)
And one another version of her angry dance adds that Lord Shiva was lying among the corpse cautiously. He did so because it was the only choice left to stop her angry dance and bring her to her senses and protect the world from annihilation.
If your wife or girlfriend takes Kali avatar and performs angry dance with all yelling and screaming and if she also has any form of weapon in her hand, lie down and play dead. You will be doing that only to protect the world from annihilation.
Moral of the story : "Not just all that glitters isnt gold" but also "Not all that are blue are Smurfs or Navi from Pandora"
Check more comparison mythology stories here