Mother Hydra is the consort of Father Dagon. She rules from the underwater city of Y'ha-nthlei and is one of the two monarch ruling over the Deep Ones. It is unclear whether she is a Great Old One like her husband Dagon or if she is just a very powerful Deep One.
In game, she looks like a fat female humanoid similar to the Deep Ones, and has four digits on her limbs and two sets of breasts. She is responsible for the barrier around Y'ha-nthlei that prevents the submarines from harming the city.
She is seen inside her Temple leading prayer with her fellow cultists. Her wailing stops Jack from defeating the Deep Ones, but he eventually kills her.
Jack must use the Yithian power weapon and electrify the water leading to underneath her barrier. Killing her destroys the shield around the city, allowing the American subs to continue bombarding the city.