The Surface.

Where life and flora exist and flourish, where the fresh breeze of air is ever-
present.

It's what every monster is longing for; it's what he's longing for. So much has happened, and so much has been sacrificed. Never in his life has he rested nor has he given in to hopelessness to achieve his goal in freeing his kind from this wretched prison the humans placed them in. No matter how long or how painful failure each day brings, he swore an oath to dedicate his whole life in bringing monsters once again into the light at the end of the tunnel...

The freedom they deserved in the first place.

One day, a spark of thought erupted from deep within his mind; a spark of hope. With it, freedom may not be as far as every monster would've thought it be.

Vector: http://vignette3.wikia.nocookie.net/undertale-au/images/5/55/Black_then_i_m_white_by_zarla-dam0qpa.png/revision/latest?cb=20161205005143