The Lernaean Hydra, also known as the Hydra of Lerna or more often simply known as the Hydra, is a gigantic serpent with nine heads, with the center one being immortal. It is one of the spawn of Typhon and Echidna. With each head that was cut off, two more grew. The monster's haunt was the marshes of Lerna near Argos. The Lernaean Hydra was killed by Heracles for the second of his Twelve Labors. There were also other hydra, called hydrae, that had some physical differences. In media, a hydra was treated as a sub-species of a dragon.
Eight Hydras guard the Underground Lake in the Valley of Despair which houses the Flood Tide Orb. The jewel is required to extinguish the flames surrounding Belial's Castle, and can only be retrieved by Undine.
Shin Megami Tensei V
"One of the monsters in Greek mythology. It is usually depicted as having 9 heads, but some may say that it has as many as 100.
Its father is Typhon, giant of the wind, and its mother is Echidna, goddess of the snake. The Hydra has incredible regenerating power; so much so that when one of its heads is cut off, two more will sprout from the wound. It is also extremely venomous and simply inhaling its breath is enough to kill. The slaying of Hydra is listed as one of the 12 great labors of Hercules."
—Shin Megami Tensei V lore
Hydra is a demon of the Drake race and the first major boss encountered. It was sent to Tokyo Tower to kill angels but doesn't distinguish between the heavenly host or demons of chaos. It attempts to devour anyone entering its new territory and has become known as the demon of the tower. The protagonist encounters the Hydra will venturing to Tokyo Tower to recover some of Aogami's memories. After viewing Aogami's memories, the Hydra tries to surround and devour the protagnist, barely missing before the protagonist jumps away and prepares to fight.
The Hydra is durable, able to soak up hits from its weakness easily. It will broadcast its use of Toxic Breath by using Omagatoki: Critical the turn before. Guarding will mitigate the damage, but demons can still be poisoned while guarding, so stocking up on Amrita Soda or having a demon with Patra is recommended. Although the skill cannot instakill weak users, it can still be deadly under the enhancement of the Magatsuhi skill. The Hyrda's extra heads will fall off when roughly a third of its HP remains, and will begin to use Fire Breath more frequently in addition to Frenzy. If the protagonist is willing to level past Hydra a bit, Aitvaras can be obtained fused once the protagonist reaches level 17, that resists Poison and nullifies Fire attacks. A demon with Media will also make healing the party easier during the battle.
Hydra can be unlocked as an ally by the protagonist with a special fusion of Yurlungur, Orthrus and Basilisk, where it retains its Toxic Breath skill.
If Hydra is on the protagonist's party and speaks to an enemy Yamata no Orochi, Hydra will boast that it is more powerful as it has an extra head, prompting Orochi to attack it. Hydra may also speak to an enemy Cerberus, in which he reveals that he is his brother. Cerberus is delighted to see Hydra, although Hydra admits that it cannot remember theothers. Cerberus reassures that they will be able to see each other in the future and gives him an item before leaving.
Demikids Light & Dark
"A nine-eyed demon that is enormous and just about impossible to kill."