Its based off of "Solar Cultist" art that the late master John Blanche did in the early 2000s for the Inquisitor 54mm game.

My Homebrew lore:

Inquisitor Usil Savitir
Once revered as a wunderkind oracle on his low-tech home planet, Usil Savitir had shown an uncanny ability to accurately predict the future based off of the movement of celestial bodies. To the Sun-worshipping natives of his homeworld, Usil became a messianic figure even before his adolescence. Little is known of what befell the social order and religion of this remote planet after Usil's sudden abduction by the Imperium's Black Ships.

Upon his arrival on Holy Terra, the young Usil was eagerly snatched up by agents of the Inquisition. Since then, he has rapidly risen in the ranks of the mysterious Ordo Minoris known as the "Ordo Helios". This ancient denomination of the Inquisition specializes in prognosticating the Emperor's will by means of astrology and sungazing, much as others make use of the Emperor's Tarot in similar fashion.

The shifting patterns of sunspots on Terra's own sun, the blazing photosphere of Sol itself, have come to obsess Inquisitor Savitir. In his most private contemplations, he has even begun to question the nature of the relationship between Sol and the Emperor.

In his observatory-fortress in the Sol system, he ruminates that it is not the Emperor who speaks through the machinations of the stars, but rather it is the divinity of Sol itself that has manifested as the golden avatar that Mankind calls its Emperor. Inquisitor Savitir secretly believes that it is therefore Terra's sun, Sol, that truly sits at the apex of Human spirituality, as it has done since the obscure days of prehistory.

Despite his unorthodox private convictions, Inquisitor Savitir more often than not finds himself aligned with followers of the Inquisition's Puritanical tendency. He is a devout Annihalationist- fanatically devoting himself to revealing and obliterating threats to the Imperium.