The Ruins Greatsword is one of the nine legendary armaments in Elden Ring which is obtained after defeating the dual boss fight featuring a Misbegotten Warrior and a Crucible Knight in the Redmane Plaza. In order to spawn this boss fight, you will need to defeat Starscourge Radahn in the Radahn Festival put together by Witch-Hunter Jerren. After defeating Radahn, make your way back to the church where you will find Jerren seated in his chair – talk to him and completely exhaust his dialogue. Then, respawn into the area by traveling to the Chamber Outside the Plaza Site of Grace, after which you will find the Plaza gated away by a fog gate, behind which the duo boss fight will take place.