Bhatt Kal Sahar was a Sikh Brahmin bard in the court of Guru Arjan, whose 54 hymns are incorporated in Adi Granth. The title Bhatt is given to learned Brahmins. Traditionally, Kal Sahar is believed to a collector of hymns of other Bhatts which were later incorporated into Guru Granth Sahib by Guru Arjan.
References
Bhatt Kalshar Wikipedia(Text) CC BY-SA