From the first episode of Tower of God, fans are introduced to Princess Ha Yuri Zahard and the 13 Month Series of weapons, particularly Black March and Green April. These weapons are special and were effectively portrayed as so, as they are only granted to a select few in the story. Apart from being some of the strongest weapons in the series, they are also integral to the plot of the story and hold the key to the inaccessible 135th level of the Tower.