They're part of the Astartes Praeses - a group of chapters founded to guard the EoT - along with the relictors among others.

They acted in the 13th crusade and committed all but one reserve company.

Their Chapter Master, Maxim Absolon was MIA, after rescuing an astropathic team from a traitor guard push towards Belis Corona. His T-hawk was shot down but the majority of his 1st company escaped.

His 2nd in command - Captain Raoul took over and took command of the chapter, distancing them from Logan Grimnars command.

The chapter was based on a star fort that traveled between 3 systems surrounding the eye. Cadia, Belis, Corona Nemesis Tessera.

The chapter continues to fight in the war despite Absolon being MIA, though they continue to search for their lost master. Many thousands of years after their chapter master was lost they received a message from an inquisitor on Nemesis Tessera by the name of inquisitor Aesop, he claimed that he had found Absalon on a remote half destroyed moon that had been separated from its planet long ago, and as any chapter would they mobilized to extract him.

Inquisitor Aesop and the acting Chapter Master made planet fall on the destroyed moon, planning on extracting Maxim but what they found was far from expected. Inside the wreck of his downed thunder hawk they found him. Absolon has been taken over by some form of unknown alien technology and was almost completely unrecognizable. Aesop being a radical, and thus interested in Absolon's transformation decided not to kill him but to bring him back to Nemesis Tessera for study and to possibly use him as a weapon to defend the chapter when times are at their hardest.

Aesop as a Radical, researches the effects that the alien tech has had on Absolon. During his investigation he discovers that Absolon memories and personality are completely intact and that he still desires to work for his chapter. Captain Raoul reluctantly steps down form acting chapter master and lets Absolon retake command of the chapter. Absolon decides that the chapter has strayed to far from the teachings of Logan Grimnars and decides to try to reinstate them to an extent.

Inquisitor Aesop introduces Absalon to a representative from the relictors that he has befriended and worked with in the past.

With the Inquisitor Lords blessing, the marine’s exemplar begin using reverse engineered alien tech in their fight against chaos. Much like the relictors and there use of daemon weapons. The inquisitor and the chapter being to work closely together, experimenting with integrating the alien tech into their armor, weapons and vehicles for use on the battlefield.

The machanicus of mars has stared to take a keen interest in the Marines Exemplar and their use of the alien tech that they have discovered during their campaign, however, they are on good terms with the local inquisitors and specifically the inquisitor lord aesop and as such are afforded a certain amnesty that other chapter are not privy to. You could say that the chapter is somewhat taken under the wing of the Inquisition and thus given much leeway, Similar to the deathwatch/grey knights, but unofficially and with a strong feeling of suspicion.