The Legend of Korra - Season 4 Episode 4 The Calling Only the Avatar can master all four elements.
Overview: While the young airbenders look far and wide for Korra, Ikki is briefly captured by two Earth Empire soldiers. Based on what they say, Ikki leads the trio to the swamp. There, Korra is still haunted by visions of being hurt by her past enemies, but manages to connect to the siblings through the great banyan-grove tree's roots. After they beg her to resume her Avatar duties to face Kuvira, Toph's advice helps Korra to let go of her fears, bend the rest of the poison out of herself and re-enter the Avatar State.