Tower of Heaven

The Tower of Heaven (楽園の塔 Rakuen no Tō also called the R-System) is a special structure, hypothetically allowing the restoration to life of any person.

Description
Tower of Heaven or Resurrection-System as the rumors said, allows any person to restore to life any other human. Still, to completed the process need (apart from Tower itself) a enormous magic power, about 270 Million Edeas, which fund it, only thing, produced that kind of energy is Etherion, a weapon owned by Magic Council. But, not only power is needed, the most important element of all is to gain a "Sacrifice", person whose magic power is on the same (or it's very close to) level as the person, who would be revived.

History
It was a structure that many dark mages tried to make in order to revive the dark mage Zeref. The Mages captured many slaves in hope of using them to complete the tower, using terrible methods to keep the slaves in line.

Once, a slave named Erza Scarlet was punished (though Shou was at fault) for leading an escape attempt, she was tortured to made an example of even losing her right eye in the process. Another child slave, Jellal Fernandes, came to her rescue and saved her, but was captured instead. With this, Erza led a revolt to save him which was successful but Jellal had changed thanks to Ultear'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.

Tower of Heaven arc
Years later, Jellal had completed the tower and needed a great magic power (exactly 270 million Edeas, from firing Etherion) and a sacrifice (he chose Erza) to revive Zeref.

After getting the energy from Aetherion, the Tower of Heaven transformed into it's complete form, a giant Lacrima crystal. It was destroyed by giant explosion, after Natsu Dragneel damaged it.