The Siblings

Long ago.
Two races ruled over the earth: HUMANS and MONSTERS.
One day, war broke out among the two races.
After a long battle, the HUMANS emerged victorious.
They sealed the monsters underground with a powerful spell.
( MT. EBBOT, 201X. )
It is said that whoever crosses the barrier can never return.
Will Chara be the one to prove this wrong, or will they make this worse?