From Halopedia, the Halo wiki
The Oathsworn is an example of personalized equipment utilized by the Spartan-IIs, specifically Kelly-087. Many Oathsworn shotguns have existed over the years, with each being created prior to another mission after the previous Oathsworn had been lost or abandoned by Kelly on the battlefield. While each version of the weapon has its slight differences, the markings have remained the same for years. The words Kelly whispers as she rebuilds and tunes each shotgun change every time, but the intent behind them remains the same.
October 2558 Oathsworn
Halo 5: Guardians
The Oathsworn is a mythic weapon in Halo 5: Guardians available as an Ultra Rare Level 5 REQ that grants 150 REQ Points when sold. As a slightly upgraded version of the Lawgiver, the Oathsworn has increased damage, accuracy, and rate of fire compared to the normal shotgun, features slightly more range and accuracy over the Lawgiver, and increases the user's movement speed while the weapon is equipped. As the weapon has a built-in Speed Boost, users of Oathsworn receive white auras and highlights, making them more easily visible on the battlefield.
In the campaign, the Oathsworn can be equipped only if Kelly is selected by a player in cooperative play. Because of this, the Oathsworn is the only special weapon in the game that cannot be used by the player in the solo campaign. The campaign rendition of the Oathsworn has the Speed Boost removed and is exactly identical performance-wise to a standard Shotgun, including the strong vertical recoil.
Halo 5: Guardians REQ cards
List of appearances