Mantra as visual

In chapter four of the Bhagavad Gita, Lord Krishna tells
Arjuna that He has had many avataras previously, and Arjuna too has had many births in this world. The Lord remembers all His avataras, but Arjuna does not. Perhaps, here the Lord has in mind His previous incarnations as Narayana and Nara, said M.K. Srinivasan, in a discourse.

The Supreme One took birth as twins, called Nara and
Narayana, who became sages, and preached the means for
liberation. Even today, the twin avataras can be worshipped
at Badrikasrama. Nara and Narayana were exponents of
Ashtakshara mantra. The Hayagreeva Sahasranama Stotra
also talks of Nara and Narayana as pupil and teacher.
These two-part manifestations of the Supreme One
appeared in Dvapara Yuga as Krishna and Arjuna to relieve
the Earth of its burden. Krishna tells Uddhava that He took
the form of Sage Narayana and asks him to meditate on Him
at Badrikasrama. When she is being disrobed in
Duryodhana’s court, Draupadi calls out ‘Nara Narayana’.

Thus there are many references to Nara and Narayana. And
now Krishna (Narayana) teaches Arjuna (Nara) the
Bhagavad Gita, because he is a fit recipient. The Lord shows His Viswarupa form to Arjuna. The time of death of everyone, including those Arjuna hesitates to kill, has been marked already. Arjuna will be merely an instrument. Arjuna sees everyone caught in His jaws, as their
lives are being brought to an end. The Viswarupa is like a
visual representation of the Ashtakshara mantra, and
Krishna is showing Arjuna the significance of the Mantra.
Thus Lord Narayana is also the One who destroys, and He
Himself says to Arjuna that He is Kala, the destroyer.