Eisenstein

The Eisenstein Incident
During the opening days of the Horus Heresy, there were still a handful of marines in the traitor legions who were loyal to the Emperor, rather than the ruinous powers of Chaos. Among them were a cadre of Death Guard. They escaped during the Istvaan Incident in a small frigate, the Eisenstein. Their goal was to warn the Emperor of the tragedy that had befallen the Imperium, and the massacre of their fellow marines in what became known as the Dropsite Massacres.

Stalled
Having narrowly escaped utter destruction at the hands of Horus' massive fleets around Istvaan, the Death Guard, probably less than a hundred in number attempted to jump into the Warp. However, their only navigator had been grievously wounded in the escape and they could only stay in the Warp at great peril. So their leader, Captain Garro, struck upon a solution: They would detonate their warp drives, hopefully a passing ship would take them to Terra.

To the Moon
The plan worked and Rogal Dorn of the Imperial Fists and his chapter picked up the ship. While initially reluctant to believe Garro, faced with overwhelming evidence from multiple sources, including a member of the Luna Wolves, he eventually relented (probably saving his chapter from destruction at the hands of the traitor marines) and the navigators of Dorn's ship made their way to Terra.

Inquisitors
After arriving in system with the news (which the Emperor had no doubt already found out by virtue of his psychic powers) Captain Garro, his men and a member of the Luna Wolves who had miraculously escaped his traitorous brothers in the Istvaan system were placed in a fortress on the moon. One of the Regents of Terra met with them after the fighting and told them that the Imperium had need of men with such strength and "inquisitive natures" as Garro and his men exemplified. For this reason, and their struggle against one of their own who had been consumed by Nurgle (likely in the same manner as Typhus) it is highly likely that Garro, his men and Captain Qruze of the Luna Wolves became the founders of the Inquisition.