The Dark Mother is a deity worshipped by Abigail Cockbane, as well as the character of ContraPoints.
The Dark Mother is used to parody biological essentialism, being a mysterious, all-powerful feminine force that TERFs worship.
The Dark Mother is portrayed by a photo of Anita Sarkeesian. This satirises the obsessive hatred towards Sarkeesian in right-wing circles.
The Ocean is referred to as the Dark Mother. It is possible that this is one of the Dark Mother's forms.