When the star can no longer support itself, the core collapses completely, forming a stellar-mass black hole, and consuming the nascent supernova without having the massive explosion.
For a distant observer, the red supergiant star will seem to wink out of existence with little or no flare-up.
Failed supernovae are one of several events that theoretically signal the advent of a black hole born from an extremely massive star, others including hypernovae and long-duration gamma-ray bursts.
Theoretically, a red supergiant star may be too massive to explode into a supernova, and collapse directly into being a black hole, without the bright flash.
This process would occur in the higher mass red supergiants, explaining the absence of observed supernovae with such progenitors.