any ideas what in the third panel will be?

Tale of the human who fell into the underground world of monsters beneath mount Ebbot.

thats literally the game lore dude..

The Whole plot

Exactly

neat-o

I mean, he is wrong, since Sans is one of the least important characters in Undertale.
He's the only one of the 'main' characters who undergoes absolutely no change throughout the plotline.

is undertale a game about sans life?

no

To be honest, Undertale would be a lot better without Sans.
The only thing Sans ever did in the Pacifist Route was save your life twice.
Everybody else was way more helpful.

yes.

Here's one for you:
"Legend featuring the world below the mountain."