Like a Brahmarshi ( a devotee of the first rank and order) he was born among men and is hard to be vanquished even by the gods. There is no slayer of Bhishma, Oking.
ब्रह्मर्षिसदृशो जज्ञे देवैरपि सुदुःसहः। न हन्ता विद्यते चापि राजन् भीष्मस्य कश्चन॥