Breaches were inherently unstable, so even if they were opened by cultists or a necromancer, they would eventually collapse without the direct intervention of said cultists, necromancer, or…something else. Hence, the boss of a breach arena. Without the cultists or necromancer, the breach could only stay open with a Class 5 Breacher keeping it open, so you kill the boss, you cut off the breach’s energy supply, and the rift closes. End of story…