What explosive event signals the end state for a massive Red Supergiant?

Type II supernova.

The death of a red supergiant is marked by one of the most energetic events in the cosmos: a Type II supernova. This occurs because these massive stars possess enough internal pressure and temperature to sustain fusion of progressively heavier elements in their cores, building up layers like an onion, all the way up to iron. Since fusing iron actually consumes energy rather than releasing it, once the inert iron core forms, the outward thermal pressure instantly vanishes. Gravity wins catastrophically, causing the core to collapse in mere seconds. This collapse rebounds off the newly compressed material, triggering a massive shockwave that blasts the star's outer layers into space in a spectacular explosion known as a Type II supernova, an event so luminous it can briefly outshine an entire galaxy.