A hydra wand is any wand that has a small slice of dried sucker from a kraken tentacle in its core.
Kraken
A kraken is a sea monster in the form of a gigantic squid that can occasionally be found off the coast of Norway. Most of the time it resides on or near the ocean bottom, but occasionally it has been known to rise to the surface, attack a ship, and eat its sailors. The kraken is itself rare and it only rarely attacks ships, making it quite difficult and expensive to obtain a piece of a kraken's arm. The wand maker who intends to make a kraken wand must beware of sailors attempting to sell the dried suckers of ordinary squids and octopi instead of a kraken. Only wands containing a tiny strip of actual dried kraken sucker are capable of strengthening dark water spells.
Kraken
Magica
