Kingdom Hearts II
Hades sent the Hydra to destroy the Olympus Coliseum and kill Hercules. Hercules had to fight it in the Coliseum and believed he had defeated it, but it was in fact was merely playing dead and started to destroy the Coliseum. The Hydra met its match when Sora, Donald, and Goofy as well as Phil, Pegasus, and Megara came in to help out. However, the aftermath of the battle was rather unfavorable—the Coliseum was destroyed and Hercules lost his hero's glow, making him later fall prey to the plans of Hades.
The Hydra is a massive, reptilian creature with dark purple skin. Its body is rather bulky and it crawls using two four-fingered hands with white claws. It has a long, thin tail and each time one of its heads are cut off, two more grow to replace it. Its number of heads can apparently grow exponentially in Hercules, though in Kingdom Hearts II its maximum is seven. All of the Hydra’s head are identical to each other, each sporting the same long, serpentine neck, large mouth filled with white fangs, orange eyes with slit pupils, and spikes lining the top of their heads down the length of their necks.
When one of the Hydra's heads is cut off, it spews green blood. In the International releases, however, this is desensitized to be purple smoke instead, though it remains green blood for the initial cutscene.
The Hydra's name references the Lernaean Hydra, a serpent-like water monster of Greek mythology. It originally had nine heads that would replace each other in the same manner as the Hydra in Hercules, and was killed by Hercules as part of his Twelve Labours.
In this battle, you have the most character support of any battle. Donald and Goofy in your party, Phil tossing the pots to stun the Hydra, Meg giving you HP, MP and Drive orbs and Pegasus allowing you to use the Pegasus Run reaction command.
The battle begins as Sora attacks the main head for a while. Then a Reaction Command called Vanquish appears—doing so will allow Sora to remove its head. It puts its head in the sand and three heads sprout from the surrounding area. With Phil's help you can immobilize the heads with a Reaction Command called Phil One-Two and then follow it up with Urninator. You must then defeat and Vanquish the heads before they awake.
The Hydra then pulls its neck out of the ground and shows that it has seven heads. While fighting the seven heads, you can use Pegasus to fly up to the heads to strike them with a Reaction Command called 'Pegasus Run'. Once it is out of HP, you can use the Vanquish command again. If you ever run low on HP, MP, or Drive, Megara is found at the outside the arena itself, hiding on the very left side of the arena. Simply run around the area, and she will toss you what you need.
Hydra is capable of summoning dark orbs, using lightning, and using blue fire similar to Hades. At the end of the battle you get Thunder and after the cutscene, Hero's Crest. If you are defeated, you have the option to play as King Mickey.
 Reaction Commands
Phil One-Two: When the Hydra has its head in the ground, follow Phil's advice and "Get up on the Hydra's back!". When you do this, this reaction command is available. Phil will throw an urn into the air, and Sora will leap into the air.
Urninator: After Phil One-Two, use this Reaction Command. Sora will hit the urn downwards with his Keyblade, and it will smash after it lands on the Hydra's back, stunning it for a short time.
Pegasus Run: When this Reaction Command is activated, Sora will mount Pegasus, and they will fly at the Hydra's heads.
Attack: During Pegasus Run, use this Reaction Command to attack the Hydra's heads with the Keyblade.
Vanquish: When you have depleted the Hydra's health bar to zero, it will slump over. Use this Reaction Command, and Sora will slice quickly through the Hydra's necks, defeating it.
The Hydra originally appeared in the 1997 film Hercules. Hades had sent his two minions, Pain and Panic, to disguise themselves as little boys and pretend that they were trapped under a boulder, so Hercules would help them by removing the boulder, but in truth the Hydra was also hiding behind that boulder. So, after some struggle, Hercules was swallowed by the one-headed Hydra, but he managed to escape by cutting its head off from the inside. However, the Hydra grew back three heads and continued to attack Hercules. Hercules kept on trying to cut off the Hydra's heads, but soon there were dozen's of heads on the Hydra, forcing Hercules to cause a avalanche of rocks to crush him (though he survived) and the beast, slaying the Hydra.
In the Greek myths, Hercules was sent to kill the Lernaean Hydra as one of his Twelve Labours. This Hydra started out with nine heads instead of one and could breath poisonous gas. Hercules, like in the movie, tried to slay it by cutting off its heads, but, also like in the movie, this proved ineffective as it kept on growing back heads. Eventually, Hercules figured out that he couldn't just keep on chopping off the Hydra's heads. He got a fire brand from his charioteer and seared every one of the Hydra's necks after cutting off one head so they could not grow back. Eventually, Hercules had cut off each head and seared their necks and had defeated the Hydra.
 Notes and references