मनु स्मृति - यमव्रतम्

यथा यमः प्रियद्वेषौ प्राप्ते काले नियच्छति ।
तथा राज्ञा नियंतव्याः प्रजास्तद्धि यमव्रतम् ॥

- मनु स्मृति

A king must punish the guilty even if they are his own relatives. Much similar to the way how Yama (Death) takes away a person as soon as his time is over. This penance of impartiality expected from a King is known as Yamavrata (penance of Yama).

- Manu Smriti