SB 9.4.28
tasma adad dharis cakram
pratyanika-bhayavaham
ekanta-bhakti-bhavena
prito bhaktabhiraksanam
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Being very pleased by the unalloyed devotion of Maharaja Ambarisa, the Supreme Personality of Godhead gave the King His disc, which is fearful to enemies and which always protects the devotee from enemies and adversities.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
A devotee, being always engaged in the service of the Lord, may not be expert in self-defense, but because a devotee fully depends on the lotus feet of the Supreme Personality of Godhead, he is always sure of protection by the Lord. Prahlada Maharaja said:
naivodvije para duratyaya-vaitaranyas
tvad-virya-gayana-mahamrta-magna-cittah
(Bhag. 7.9.43)
A devotee is always merged in the ocean of the transcendental bliss of rendering service to the Lord. Therefore he is not at all afraid of any adverse situation in the material world. The Lord also promises, kaunteya pratijanihi na me bhaktah pranasyati: “O Arjuna, you may declare to the world that the devotees of the Lord are never vanquished.” (Bg. 9.31) For the protection of the devotees, Krsna’s disc, the Sudarsana cakra, is always ready. This disc is extremely fearful to the nondevotees (pratyanika-bhayavaham). Therefore although Maharaja Ambarisa was fully engaged in devotional service, his kingdom was free of all fear of adversity.