Calamus, after talking with Brightburn, finds him quite charming and intelligent despite his, well, insensitive choice of words. Calamus believes that he could perhaps get Brightburn to see the error of his ways through friendly conversation, and so agrees to accompany him to his study. Quite quickly, he finds himself uncomfortable with Brightburn's increasing intensity, doubling down now that unicorns are superior beings and the like. More than once, he tries to politely take his leave, worried that he may be getting into something he shouldn't. But before he can, Brightburn summons a portal and invites him inside, with an unsettling smile.
With the portal blocking the exit, Calamus doesn't have much choice. He fears what might happen if he tries to escape, and so he reluctantly agrees to go in.