Now you may be wondering about Undergallery as a whole well here is the full information for Undergallery
The Monsters and humans never had a war in this AU, they continued to live peacefully side by side and simply grew up with one another, So the monsters were never theoretically in the underground but when you fall into the Gallery, you fall into the “Underground” Which is a huge cavern painting which replaces the Ocean of the deep painting in IB, There are sections of the “Underground” in the gallery named respectively in the game, The ruins, Snowdin, Waterfall, Hotland, New home and the escape painting called the “Surface”
Instead of Roses which act as your health and life in Ib, You will find what are called Soul charms, These charms are heart shaped, made of a clear, smooth, glass like material which pulses and glows with color, You wear them around your neck, When damaged these charms will crack and if Death occurs it will shatter into pieces, the glowing color inside leaking out onto wherever the person dies, Monster or Human
You can find Empty soul charms all throughout the gallery and Toriel even gives you one to start off with, these empty ones have no color and are what you use to /Save/ Gallery monsters, letting you bring them back to the “Surface” with you
There are many different routes in the way of how the game ends, just as in Ib there is many different endings, A few are; A happy family, Locked away, Alone together, Sacrifice and a few others