Bhishma said Achala and Vrishaka are two Rathas on your side who are hard to vanquish; both the brothers, united together, will destroy your enemies.
भीष्म उवाच अचलो वृषकश्चैव सहितौ भ्रातरावुभौ। रथौ तव दुराधर्षी शत्रून् विध्वंसयिष्यतः॥