Trasadayu was a Vedic seer. Trasadasyu literally means one of whom the wicked ones are terribly afraid (trasyanti dasyavo yasmat sah). He was a royal age, who is probably the most celebrated king in the entire Rig Veda. In fact, he is the only king who was termed ardha deva (demigod). There are many references to him throughout Rig Veda, in mandalas I, IV, V, VII, VIII and X. His father was King Purukutsa and hence he was also known as Pourukutsa. Trasadasyu’s mother propitiated seven rishis (sages) in order to beget an illustrious son who could succeed his well known father. The rishis's advised her to worship the deities Indra and Varuna, by whose grace Trasadasyu was born.
The uniqueness of Trasadasyu is that not only was he the rishi (seer) of some Vedic suktas but also was the devata (deity)for a sukta (42d sukta of the 4th mandala). This sukta, for which Trasadasyu was the seer, contains ten riks. Of these, Trasadasyu is the devata of the first six verses. For the remaining four verses, Indra and Varuna constitute the deities. In that sukta, Trasadasyu declares himself as the king of two empires, o kshatriya race as well as the gods, and as the destroyer of evil forces. He also announces himself as Indra and Varuna and states that he animates all beings, creates the universe and sustains the earth and heaven. The warriors invoke him during battles, and he helps them by smiting the evil forces into dust. Trasadayu’s assertion is strikingly similar to that of Rishi Vamadeva (in Brihadaranyaka), who identifies himself with Surya and Manu. After describing the glorious nature of his Self, Trasadasyu switches over to eulogizing Varuna and Indra and sings their glories in the last four riks.
According to some scholars, Trasadasyu does not refer merely to a single king but to a great dynasty encompassing more than a hundred kings, including some from the Ikshavaku dynasty.
Besides, Trsadasyu can be philosophically viewed as an aspect of Self, which when activated, would drive away the evil elements. The warriors by invoking that spirit would become empowered to vanquish the evil forces, external as well internal, and emerge victorious.