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After Morrighan severed the remaining Hydra heads, Izroth, Sacred Blade, and the others focused on eliminating the massive influx of more than 500 skeletal warriors that spawned from the Hydra's teeth.
While the skeletal warriors were numerous, in the end, they were no match for the party without the Hydra's back-up.
As for the Immortal Hydra, once the last of the skeletal warriors were taken care of, its large body sunk back into the lake. However, even after being severed and having its HP fall to 0%, the Hydra's main middle head did not die! And, no matter what attack they threw at it, the middle head was immune to their damage.
The middle head's gaze locked onto the party just as it sunk back into the lake along with its body.