The Zoroastrian deity of covenants and light is Mithra.
Mithra, written Miθra in Avestan, is a yazata—a divinity worthy of worship—associated with covenants, oaths, contracts, light, justice, and friendship. Because agreements were essential to social order, Mithra was believed to watch over promises and punish those who broke them. He is also portrayed as an all-seeing guardian of truth and a protector of the moral order called asha.
The Avesta’s Mithra is not identical to the later Roman god Mithras, although the Roman mystery religion borrowed or reinterpreted Iranian imagery and names. Ancient Iranian Mithra has strong connections with the sun, open spaces, judgment, fertility, water, and the protection of cattle and communities. The tenth Yasht, known as the Mithra Yasht, is the principal Avestan hymn devoted to him.
Rashnu is another figure connected with justice and judgment, while Verethragna represents victory and Atar personifies sacred fire. Those associations can make the options seem related, but the combination of covenants and light points specifically to Mithra. His name is also related to the Indo-Iranian deity Mitra and the Roman Mithras.