Tower of Heaven

the Tower of Paradise was a structure that many dark mages tried to make inorder to revive the dark mage Zeref. They captured many slaves in hopes of using them to complete the tower and they used terrible methods to keep the slaves in line.

Once, a slave Erza Scarlet was punished for apparently leading an escape attempted (which she diddn't) and she was tortured to made an example of even losing her right eye in the process. Another child slave, Gerard, cam eto her rescue and saved her but was captured instead. With this, Erza led a revolt to save him which was successful but Gerard had changed thanks to Zeref's influence and kept all the slaves at the tower (treating them much better) and he finished the tower as he sent Erza away to freedom.

Years later, Gerard had completed the tower and needed a sacrifice to revive Zeref, he chose Erza to be his sacrifice.