Trylla performs the Changeling Spell
The changeling spell is a type of magic which only trolls can preform used in both the graphic novels and animated series.
The spell magically switches a troll child with a human child, transforming each child into a member of the other species in the process. The human child will become a troll, complete with all the powers and weaknesses trolls have, while the troll child becomes human. They also physically swap locations, with the human child being transported to where the troll child was at the time the spell was cast and vice-versa. Upon being enchanted by the spell, that human child will loose all of his or her clothing in which case, thus becomes the troll's possession.
In both the graphic novels and the animated series, Trylla performed this spell on her daughter Baba to have her swap places with Hilda. To do so she took two stone figures of Hilda and Baba that Baba made earlier, and swapped their heads around.
The only known way to reverse the spell is to return both human and troll child to their real mothers. This is a fact even Trylla herself was unfamiliar with despite knowing how to perform the spell.