As you enter the true lab, you walk for a bit then you encounter Sans with a giant machine behind him and he looks like he plans on doing something with you with that machine but you don't know yet...

Sans: Heya... there's been a change of plans. There's something I want to talk to you about. Kid, I'm tired. Why don't ya just relax and load off a bit? It's way easier. But we both know that isn't gonna happen any time soon... you'll never be happy, will you? Welp, you probably know what this machine does behind me... you probably can presume what's going to happen next... You'll be dead where you stand.

Sans then suddenly pulls out a tablet as his eyes go pitch black, that machine behind him activates and it eye sockets light up a ominous white circle color as it extracts half of your determination soul.

Sans: Lets get started.

and with that the battle has started.