I think it's Sword Logic, actually. To become stronger, we kill, and take the strength of the dead as our own. We killed strike bosses and gained strength, killed The Templar, Gatekeepers, and Atheon. We then fought Ir Yut, and gained her strength. Eventually, we killed Crota. We then moved on to kill PoE bosses. Then Skolas. After that, we struck down Oryx in our world. Later, the Warpriest, Golgoroth, and the Daughters fall to us. By the time we reach Oryx we've killed so much (Not only that but there's also 6 of us) that by Sword Logic, we'd be nigh unstoppable.