What catastrophic event immediately follows the formation of an iron core in a very massive star?

Gravity catastrophically wins, causing instantaneous collapse leading to supernova.

For stars significantly more massive than the Sun, the relentless gravitational compression continues to force the core to fuse progressively heavier elements—carbon, neon, oxygen, and eventually iron. The formation of an iron core marks the absolute endpoint of energy generation via fusion. Unlike lighter elements, fusing iron does not release energy; rather, the process consumes energy. When the iron core forms, the outward pressure generated by fusion instantly vanishes. With no counterforce remaining, gravity achieves absolute, catastrophic dominance, leading to an instantaneous collapse of the core. This runaway collapse is violently halted by quantum resistance, causing a massive rebound shockwave that triggers a supernova explosion, leaving behind either a neutron star or a black hole.