Its den is in swamps between forest and the source of a great river.  From there is raids the surrounding lands, feeding on the livestock and killing crops with it's poisonious breath.

In ancient times, Hercules, along with iolaus was given the task of killing it. Finding that just chopping off it's head wouldn't work...for each time he did, the hydra would grow 2 more in its place.  But finding that they could cauterize the neck after each head was dislodged, Hercules was finally able to defeat this massive creature.  He then used the blood of the Hydra to dip his arrowheads in since it was such a deadly poison.