Prahlada and Holika
SatyugaPrahlada, a devotee of Vishnu, refused to stop worshipping him despite his demon-king father Hiranyakashipu. His aunt Holika, immune to fire, sat with Prahlada in fire to burn him, but divine protection saved Prahlada and burned Holika instead. This is commemorated as Holika Dahan.
Vishnu Purana; Bhagavata Purana (Narasimha avatar)