(When you entered the castle and while roaming around, Mario walked by you with a remorseful look in face but, when you try to interact with him, he just looked at you with that same look but more sadder and he just leaves the castle through the door. And so you continue adventuring through the castle)

(You stumble upon a similar room that looks like where Mario fought Bowser but a certain of it is not there, so you try to leave the room until some person climbs up from the lava. When they get back to the surface, they appear to be Luigi but burned up from the lava he got dropped in after his battle with Mario)

(Luigi walks right infront of you)

"Move away this instant. I've got a family reunion that i'm late too, but if a conversation is what you're looking for. Make this quick"