Amma is the mother and grandmother of all Trolls. She is currently sleeping deep underneath the city of Trolberg. She appears in the graphic novel "Hilda and the Mountain King", along with its movie adaptation.
She is only ever seen in silhouette, so how exactly she looked is unknown.
Long ago, Amma built the mountains around Trolberg for her children to reside in. Afterwards, she went to sleep between the mountains, surrounded by her loved ones. While she slept however, her body got covered by earth, and humans build a settlement on top of it. This settlement grew into the city of Trolberg.
As the city grew, her children were driven further and further away from her, but still she didn't want to rise up and destroy the city. Since her voice could barely reach the other trolls anymore, most of them forgot about her. Only the oldest of trolls, like Trundle remembered her. Still, her presence kept drawing Trolls towards Trolberg, even though they could no longer remember what it was that attracted them, and knowing fully well that humans hated and feared them.
Trundle planned to lead the other trolls in an attack against Trolberg, hoping that, if the humans fought back and thus put Amma's children in danger, she would finally interfere and destroy the city. The plan was thwarted however, and instead the trolls were allowed to come into Trolberg one night a year to be close to Amma again. This celebration became known as The Night of the Trolls.