From Halopedia, the Halo wiki
Reckless Pursuit was created by the Sangheili later titled Pelahsar the Strident, as a last-ditch attempt to win the ancient war with the San'Shyuum. During the war, the San'Shyuum had successfully been able to employ the keyship Anodyne Spirit to great effect, effectively leading to the destruction of dozens of Sangheili colonies. As the dreadnaught bore down on Sanghelios itself, many began to question the Sangheili religious taboo on studying Forerunner relics simply to survive the onslaught - an endeavour championed by Pelahsar's father and then-First Kaidon Mhelaat 'Orlam at the ultimate expense of his own life.
Mhelaat's assassination did not deter his son, and Pelahsar set about continuing his father's work - ultimately creating Reckless Pursuit as an experimental weapons test outfitted with Forerunner-derived weaponry. The initial tests were successful, ultimately leading Reckless Pursuit to be deployed in the dreadnaught's assault on Sanghelios - successfully downing the gargantuan ship and forcing the San'Shyuum to consider peace negotiations. This act forced the signing of the Writ of Union between the two groups, ultimately causing the formation of the Covenant empire as a whole.