Whenever the head of a lernaean hydra was removed, two more heads would quickly regenerate from the stump, though this could be prevented by applying fire or acid to the injured neck stump. In ages past, this ability was apparently unique to the Lernaean hydra, but later all hydras possessed this trait.
However, Lernaean hydras could only support an amount of heads that was exactly twice that of their original number of heads. Any heads beyond that amount would wither away and fall off within a day.[note 1]
Until all heads were removed from their body, a Lernaean hydra could not die. The spells disintegrate, finger of death, and slay living were some of the few known means to bypass this creature's near-invulnerability and kill them outright.
A fifty-headed Lernaean hydra was said to have been slain by the giant goddess Hiatea in order to prove herself to her father, Annam All-Father. She was said to have used her enchanted spear to slay it, preventing the regeneration of its heads by cauterizing them with fire.
In the mid–14th century DR, a Lernaean hydra was known to lair within a hidden room of Undermountain's third floor. Around 1372 DR, a six-headed Lernaean hydra roamed the valley of High Thorog after having been freed from Telkoun's Tower.
Like most hydras, Lernaeans lived solitary lives and only gathered for the purpose of mating.
Like the standard hydra, these creatures were typically found in marshes, swamps, and underground lairs. In particular, they were known to inhabit the Umber Marshes in the Unapproachable East. They were also known to be found in the marshes of Cormyr, including the Farsea Marshes and the Tun Marshes.
- The ability to regenerate heads was the sole defining feature of Lernaean hydras between 1st and 3rd edition. In Monster Manual 3.5 edition, this trait was given to all hydras and the Lernaean was thus excluded from the book.
- Skip Williams, Jonathan Tweet and Monte Cook (October 2000). Monster Manual 3rd edition. (Wizards of the Coast), pp. 122–123. ISBN 0-7869-1552-1.
- David "Zeb" Cook et al. (1989). Monstrous Compendium Volume One. (TSR, Inc), p. 58. ISBN 0-8803-8738-6.
- Gary Gygax (December 1977). Monster Manual, 1st edition. (TSR, Inc), p. 53. ISBN 0-9356-9600-8.
- Frank Mentzer (January 1985). “Ay pronunseeAYshun gyd”. In Kim Mohan ed. Dragon #93 (TSR, Inc.), p. 28.
- Gary Gygax (December 1977). Monster Manual, 1st edition. (TSR, Inc), p. 54. ISBN 0-9356-9600-8.
- Skip Williams, Jonathan Tweet, Monte Cook (July 2003). Monster Manual v.3.5. (Wizards of the Coast), pp. 155–156. ISBN 0-7869-2893-X.
- Carl Sargent (May 1992). Monster Mythology. (TSR, Inc), p. 73. ISBN 1-5607-6362-0.
- Ed Greenwood (1991). “Campaign Guide to Undermountain”. In Steven E. Schend ed. The Ruins of Undermountain (TSR, Inc.), pp. 91–92. ISBN 1-5607-6061-3.
- Ed Greenwood and Jason Carl (July 2002). Silver Marches. (Wizards of the Coast), p. 134. ISBN 0-7869-2835-2.
- Richard Baker, Matt Forbeck, Sean K. Reynolds (May 2003). Unapproachable East. (Wizards of the Coast), p. 94. ISBN 0-7869-2881-6.
- James Butler, Elizabeth T. Danforth, Jean Rabe (September 1994). “Explorer's Manual”. In Karen S. Boomgarden ed. Elminster's Ecologies (TSR, Inc), pp. 20–21. ISBN 1-5607-6917-3.