What exactly happened to Alphys and Grillby?
Alphys, in the wake of Sans’ debilitating injury and the kingdom’s collapse, was left alone with the task of finding a way to either break the barrier (through the use of the siphon machine that she and Sans had been working on) or to defeat Flowey, which she knew was impossible. Overcome with guilt for Flowey’s very existence and hopelessness for the overwhelming circumstances, her soul falling more and more under the control of the Hunger every day, she eventually let the starving Amalgamates out of the True Lab in the hope that it would, in some way, redeem her, and they consumed her.
Grillby left Snowdin and returned to Hotland to attempt to hole up there with his daughter, wait out the plague. He lost his mind, however, and walked into one of Waterfall’s lakes when he came to his senses enough to realize he had eaten his own daughter, effectively committing suicide.