Interesting. Justin describes the crypt in question as having been sealed for centuries, which seems incompatible with there being an underwater connection to a wellspring from which a hydra could enter. Is it possible he was simply confused about the crypt’s “sealed” status?
The whole thing is in an abandoned dwarven city
"A few centuries ago, Talhundereth was abandoned by all but a few vigilant defenders. Mind flayers took advantage of Talhundereth’s depleted state to attack; during this invasion, a collapse rocked the temple and crypt alike. Both the mind flayer invaders and the temple guardians gave up Talhundereth as lost, though the mind flayers’ psychic stain remained.
Since that attack, various bandits and monsters have squatted in the abandoned temple. Sometime after the mind flayers’ reign, aspiring lore-seekers brought one piece of the shattered obelisk into Talhundereth’s chapel and another into its crypt, hoping that studying the pieces in close proximity might reveal more about the obelisk’s previous powers. That group was twisted by the psychic magic resonating in the crypt. The lore-seekers lost all interest in scholarship and abandoned the fragments.
The most recent faction to enter Talhundereth is a gang that calls itself the Cult of the Obelisk. These cultists were drawn to the broken obelisk, responding to a psychic call to serve a greater force. [...]"
The part this is about lies beyond a door that the hydra would not pass, but it is not inconceivable that others entered the room since then, everything that is needed is not that far away and the chars clearly managed to...
Here is the read-aloud text for the room with the hydra
"Most of this room has collapsed into a flooded pit. The dark water ripples and splashes strangely, as though something lurks deep within it"
and the rest of the room entry
"The collapse that occurred here long ago disturbed an underground wellspring, flooding the whole area.A hydra named Grandlejaw dozes in the water, keeping one or two heads alert for danger. The source of the splashing is a high-strung water weird named Kellikilli. After realizing Kellikilli is inedible, Grandlejaw grudgingly decided to tolerate the water weird. The water weird’s constant, frenetic motion keeps the water bubbly, which the hydra enjoys. Kellikilli likes fighting alongside a creature as tough as Grandlejaw"
and some stuff about what happens depending on what the chars do
Says nothing about when the hydra arrived, but at a minimum the collapse was long ago