Unable to Enter Abyssal Cultist Base

I am unable to proceed through the story because I can't transform into an Abyssal Cultist and pass through the first force field.

In front of Utira Library I chose to battle Damien when provided the choice and remember having the text state that I obtained the Abyssal Cultist's Hair after defeating him.

However, there is no choice to transform into the Cultist. There are only the options for Darkai Cultist and Seed Flare.