Bald Mountain is the main set of the first half of Fantasia's last segment, Night on Bald Mountain/Ave Maria. On Walpurgis Night, the tip of the dark and morose mountain is the great demon, Chernabog. There, he summons his minions to his service at Bald Mountain. The insides of the mountain become a fiery volcano as he flings his minions within.
Just as he begins to continue his reign of pure evil, the light of day sends the Demons back to where they once were and Chernabog will always survive. Bald Mountain has always been known as the source of darkness in the Disney Franchise since the sequence in Fantasia.
Bald Mountain is home to the Chernabog, a winged demon who guards a crystal ball containing a powerful curse. Those who dare to venture in will face three fearsome obstacles in their way. Rumplestiltskin desires to acquire the curse for himself, and manipulates Cruella De Vil, Maleficent and Ursula into helping him so they can get their happy endings in life. When blocked by a whole horde of venomous bugs, Cruella uses her power of persuasion on one of creatures, which then influences the entire swarm into moving aside. After this, the group reach a room with the crystal ball on a bier, but it is surrounded by fire, so Maleficent absorbs the flames into her staff. Lastly, Ursula grabs the curse with her tentacles and hands it to Rumplestiltskin. From this action, the Chernabog awakens to face the intruders. Only then, Rumplestiltskin admits he used them for the sake of obtaining the curse for himself. Prior to leaving, he advises them on the Chernabog's ability to detect the heart with the greatest potential for evil and devour it. Left to defend themselves, the women hide behind rock pillars. They brainstorm a plan to allow the Chernabog to attack whoever it desires while the remaining two climb to safety and then rescue the remaining person. Once the women step out, the Chernabog looks at each of them until it lunges at Maleficent, who combats with magic blasts. After Cruella and Ursula scale a crack in the ceiling, Maleficent reminds them of the plan, but when they do not save her, she grudgingly accepts her end at the Chernabog's hands. As the demon swoops down at her, Maleficent expresses bitter hopes that it'll choke on her bones. Suddenly, she is pulled up to higher ground by Ursula's tentacles. The Chernabog, unable to reach them, settles for letting out a vicious roar.
Since Chernabog's minions appear to dance inside the mountain itself, it's possible Bald Mountain is either hollow or actually a volcano with Chernabog stuck in the crater and covering it.
In Slavic folklore, "bald mountain" (or Lysa Hora in those locations) is a generic name shared by small mountains or hills with no tree on their tops, and they are believed to be places where witches gather to perform their rituals.
The real Bald Mountain which inspired Mussorgsky is the Lysa Hora near Kiev, Ukraine.